When Sally Shelton was in high school in Monett, Mo. (pop. 6,000), she was a cheerleader. Not because she particularly wanted to be-indeed, the week before tryouts she consistently dismissed the idea while her friends were feverishly practicing cheers and worrying about who would make the squad, that ultimate of American girl dreams.

The day of the tryouts, Shelton's mother picked her up at school. She mentioned it to her mother, who said flatly, "You want to try out, don't you? All you can do is try." Yes, Shelton confessed, she did want to, she wanted to see if she could do it.

She has 15 minutes to learn the cheers that others had been practicing for weeks. She made the squad.

Yesterday, at age 34, Sally Shelton was confirmed as the new U.S. ambassador to Barbados and the eastern Caribbean. She's the youngest ambassador now serving, and one of 10 women.

Ambassador. The word has a kind of unimpeachable resonance. It's a long way for milk distributor's daughter from Monett, Mo., to the Embassy residence, complete with swimming pool, in Barbados, but Sally Shelton views the journey with something approaching glee.

It's a job she's wanted since age 12, one she's worked toward through a Phi Beta Kappa college career, a master's degree at the Johns Hopkins School for Advanced International Studies, a Fulbright fellowship, six years as Lloyd Bentsen's foreign policy aide, and 14 months as a deputy assistant secretary of State for Inter-American Affairs.

It's a job she almost got two years ago, when she was nominated to be ambassador to EL Salvador, but was torpedoed by a combination of old-line Foreign Service opposition to political appointees and a volatile situation in EL Salvador that called for a more experienced hand.

One of the unspoken difficulties in seeing her as an ambassador may be her looks-"We know it's sexist to ever think about it," said one opponent to her appointment. Always fashionably and flatteringly dressed, with shoulder-length brown hair, green eyes, and a slim figure, she does not look like the traditional pinstriped diplomat. She is probably the first ambassador to be 5 feet 3 inches tall and resemble Natalie Wood.

They have criticized her youth, however, and hinted that this is a convenient appointment, both to boost Carter's number of women appointees and to find a job for someone who was offered an ambassadorship earlier and didn't get it. Barbados, they point out, is not one of the hot spots of the world. (Actually the temperature is a breezy 85 degrees most of the year.)

Nevertheless at this point, Sally Shelton is woman who has reached a level of success she aspired to but didn't expect to reach quite so soon. At the same time, she personifies some of the pluses and minuses of the contemporary career woman, young enough to benefit from the political pressure of the woman's movement, old enough to have experienced the double standards of being a woman in a man's world.

In fact, she's writing a book about that, about being a woman in a man's world. She has three chapters done, and hopes to work on it in Barbados, in between her official duties to seven small countries, official entertaining at least three times a week, studying German and perhaps violin, and keeping up her correspondence with a vast network of friends.

"Probably a lot of people think I'm ambitious," she said, "I don't think I'm ambitious.I consider myself driven." Symbols of Achievement

Driven, indeed. Asked what crowd she hung out with in college, for example, she said, "I didn't hang out with a crowd. I did nothing but study. I didn't go out, I didn't have boyfriends. On Friday night when everyone else in the dorm was getting ready for a date, I'd go off to the library and study some more.

"I knew exactly what I wanted out of life . . . I wanted to make Phi Beta Kappa, I wanted a graduate degree and needed a scholarship to do it, and I wanted a Fulbright."

And she wanted to get out of Monett, Mo. "I couldn't wait to get out of there . . . I was accused of putting on airs because I wanted to do something other than marry the boy next door and become a teacher. But my mother always told me - you can do anything you want, but you'll have to work hard to do it."

And she got it all (except finishing the Ph.D.).

She sat curled up in a corner of her couch, spike-heeled black slippers kicked to the side, as she paced verbally through the chronicle of her past. Her Connecticut Avenue apartment is furnished in bright, flowery prints and sharp splashes of color, filled with paintings and plants. She is also a bright splash of color, dressed in crimson, her eyes intently luminous, the gestures darting arcs of energy. There is a tension about her, a sense of being stretched tight that underlies her poise.

"The biggest thrill of my life - maybe even greater than being ambassador - was making Phi Beta Kappa. It symbolized achievement in a larger context than Monett, Mo. There I'd had everything - good grades, cheer-leader, class secretary, awards - and I felt that was easy in a small pond. Of course, as soon as I got it what I wanted was to get into a really firstrate college for my master's."

She tried to transfer from Southern Methodist University in Texas to Princeton University after her first year. "I got back this snippy little letter saying they didn't accept women. That was the first time I knew I was going to have trouble because I was a woman. I was furious." She ended up at the University of Missouri.

She was admitted to the prestigious Johns Hopkins School of Advanced International Studies, and later got the Fulbright to study in Paris. By this time she spoke Spanish, French, and Italian - all with impeccable accents.

In 1966 she took the Foreign Service exam and passed - but decided to postpone working for the government. Her decision was based in part on her objection to the war in Vietnam, and in part because "I didn't want to get lost in the bureaucracy."

Meanwhile, she'd fallen in love, with a man she later married. "It was the first time in my life I'd let myself get emotionally involved - this all sounds so trite," she said with some reluctance. It was a long courtship: he was a Mexican lawyer named Eduardo Jimenez, who was involved in his country's politics and later became their ambassador to Iceland and Norway. He now lives in Mexico and although the couple was divorced in 1976 after a long separation, they remain "good friends."

"He wanted me to be a Mexican housewife," Shelton said. "I absolutely adore Mexico, but I was terribly unhappy . . . I was young, and in love, and in essence, lost."

After separating from her husband, she sought a job with then newly elected Sen. Loyd Bentsen (D-Texas). "I just wrote him out of the blue, listing my qualifications, and it happened he needed a foreign policy specialist." she said. It also helped that an old friend of her grandfather's, Clark Thompson, had been a longtime congressman, and his recommendation was no doubt helpful.

The job with Bentsen, which threw her into association with a group of young, liberal foreign policy types, turned out to be the major break-through in her career. Because when a Democrat got into the White House, these people were ready to get jobs on the transition team and subsequently move into the State Department. It was through these contacts that Shelton came to the attention of the powers that be.

But first she had to spend a few weeks as a typist in Bentsen's office before the job opened up. "Frankly, I needed the money," she said, "but it was a mistake, and it was my fault for agreeing to do it. I hadn't spent all those years in graduate school to be a typist."

Needless to say, she is an expert typist - 98 words a minute.

"I do everything fast," she said with a touch of sheepishness. "I walk fast, I talk fast. . . it's probably nervous energy." The Shun Chetty Story

There is, of course, the dark side of success. Her divorce has left a void she readily admits, pushing her to work longer hours to elude the grasp of loneliness.

There are the minor, symbolic annoyances - being asked to present flowers to a visiting dignitary's wife, a task usually given to an official's wife (she refused), or having your secretary make a lunch reservation for "Ambassador Shelton" at a chic K Street restaurant and then seeing your male guest being greeted as the ambassador.

Then there are the rumors.

"There are always people who atribute a woman's success - particularly a woman as attractive as Sally - to her sex life," said State Department spokeswoman Jill Schuker, a friend of Shelton's for seven years. "Those kind of comments are simply occupational hazards for women."

"There are different moral standards for women," Shelton said. She doesn't expect to have dates in Barbados any more than she does here. "I don't like it, but that's the way things are."

And then there was the Shun Chetty matter.

"U. S. Envoy in Love With S. A. Lawyer," trumpeted the South African newspaper The Citizen, a pro-government organ with close ties to the secret police. It was a front-page story - all based on a tip from an anonymous letter and purported "evidence."

Shelton and Chetty, a well-known Asian lawyer in South Africa who has represented blacks in civil rights cases, were reported to have had a tryst in the "romantic Drakensberg range."

"The Citizen traced the couple to a remote part of Qwa Qwa," reported the newspaper in a breathy style that couched the alleged love affair in terms more appropriate for a nuclear accident than a piece of gossip.

"The whole thing is preposterous," said Shelton, "Shun Chetty is like my brother. He's one of South Africa's leading civil rights lawyers and I think this was an effort by the government to discredit him. It [the article] implies he's close to our ambassador and the consul in Johannesburg and implies they are CIA agents and that I'm part of that crowd."

She'd gone to Africa on vacation this winter, she said, and wanted to visit South Africa and do some hiking in the mountains. Chetty was one of several acquaintances there she contacted, and he suggested a joint excursion.

"I have frequently traveled with men with whom I am not physically or emotionally involved," she said.

They spent two days hiking, staying at a hotel where "we checked into separate rooms." After they returned to the city, Shelton went on her way and thought no more of it until Chetty called and told her about the newspaper stories.

She says the stories are full of inaccuracies aside from the main one - for example, one article described her as one of President Carter's "top women aides" while in fact she has only met Carter once. But she has learned a bitter lesson from the experience.

"I've learned my private life is not really private," she said. "I don't think I'm any big deal. But I guess because I'm a woman people look at me more than if I were a man. I knew I would be in the publice eye when I got into this, but I didn't realize that I wouldn't have any private life at all." Pros and Cons

There are of course two other questions: Is she competent, and what does the ambassador to Barbados do, anyway?

Her supporters point to her experience in Latin America, having lived in Mexico while she was married, her educational background in foreign policy and international relations, her legislative experience while working for Bentsen, her 14 months as deputy assistant secretary of State for Inter-American Affairs, her "ability to communicate," and general awareness of the sensitivities of underdeveloped nations who are in some ways dependent on the United States.

The American Foreign Service Association, which represents career foreign service officers, objected to her previous nomination to E1 Salvador and to her appointment as deputy assistant secretary.

Their objections were that she was a political appointee without the experience or qualifications to justify displacing a senior career officer from a desirable assignment. This time, however, AFSA has decided to make "no comment" according to its president, Lars Hydle.

"Many of the people who had worked with her or for her (while she was deputy assistant secretary) said she had one a good job, and that there were a lot of other political appointees who were worse," said Hydle. "We did not want to appear as though we had a personal vendetta against her. The people who were initially outraged did not feel so strongly about her youth and experience as they did two years ago."

Another critic, a former Hill colleague, says that while she "lacks depth," she is qualified to be an ambassador. "The job is more P.R. than penetrating insights into events or policy," he said. "An ambassador is a tool rather than a shaper of policy, and she's quite capable of doing the job."

Barbados' ambassador to the United States, Oliver Jackman, a man with long experience in the diplomatic world, is genuinely delighted with the appointment. "She's a marvelous choice," he said.

"A diplomat is essentially a messenger," he said in a toast to her at a recent party. "The messages have to be properly carried and truthful. She will be a very good ambassador for us."

Shelton will oversee an American staff of about 18, as well as a number of Barbadian employes. The United States has a $40-million aid program in the area, as well as military installations.

At her confirmation hearing, only two senators showed up, and neither questioned her very closely.

"I've heard nothing but good remarks concerning your qualifications and capability," said Sen. Edward Zorinsky (D-Neb.). He asked her what her duties had been while serving as the special representative to the United Nations with Andrew Young for 2 1/2 months last fall, and for her assessment of the Caribbean Development Bank. 'She's Going to be Great'

Within a few days, Sally Angela Shelton will be sworn in. She will have rented her apartment and shipped her new Triumph Spitfire (for weekends) to Barbados, and started her new role as the top American representative in the eastern Caribbean. She's never been happier in her life, she said.

Of course, she'll say in a moment when the fatigue gets to her, she wouldn't work so hard if there was a man in her life. Her mother says she works too hard; she kind of wishes her daughter would get married and have children as well as a successful career.

It's that old internal vise: being a woman helped get you your job, but sometimes it seems as though your job won't let you be a woman.

There's a pause in the discussion, which is taking place this time in a restaurant. A man stops by the table to congratulate her; he is an old professor. "Listen, she's going to be great in this job," he tells a reporter, "a lot better than some of the - - - s they've had down there."

She brightens. He says he's been trying to reach her; she promises to call him early the next week. On the way out, he introduces her to his lunch guests.

"Barbados?" they say, eyes widening in the Washington way "not bad." CAPTION: Picture 1, Sally Schelton, by Larry Morris-The Washington Post; Picture 2, Sally Shelton, by James W.K. Atherton-The Washington Post