Peter Del Giorno, a Republican candidate for Congress in Syracuse, N.Y., came to Washington this week excited at the prospect of getting professional advice on how to defeat an incumbent Democrat this fall, only to be told yesterday that he shouldn't worry about making mistakes in the campaign because he's probably not going to win anyway.

Political consultant Arthur Finkelstein had some harsh advice for Del Giorno, a 30-year-old used car salesman, and school board member, and 60 other GOP congressional hopefuls who are attending the five-day conference sponsored by the National Republican Congressional Committee.

"If any of you do get elected, and chances are 19-to-1 you won't," Finkelstein tood them, don't run your own campaign; don't let your spouse do it; and don't listen to the nice lady who praises the brilliance of your speech - she probably was the only one in the audience who thought so.

Elsewhere at the GOP workshop at the Twin Bridges Marriott, Del Girono's wife, Barbara, 29, asked a panel of congressional wives how she could protect her children, Jody, 8, and John, 5, from being exploited by being in the limelight as the members of a Congressman's family.

Again, the harsh reality of Washington life was explained, this time by Joyce Brown, wife of Rep. Clarence J. (Bud) Brown of Ohio.

"You are a celebrity at home," Mrs. Brown told Mrs. Del Giorno and two dozen other wives of nominees, "but if you win and move to Washington, it doesn't mean a thing that your husband is a member of Congress - there are 534 others.

"You won't be able to get a check cashed at the local Safeway without an identification card. I can't," said Mrs. Brown, whose husband has been in Congress more than 12 years.

It may be a tribute to the resilency of politicians that neither of the Del Giornos was discouraged by what they heard.

The Del Giornos paid their own way here, plus a $75 registration fee, to hear experts talk about issues everyone agreed inflation is the biggest one; strategy, polling, finances, speeches, and how to target voter groups such as labor, Catholics, Jews, blacks, Hispanics, chicanos, farmers and regional interests.

Frank Wolf, of Arlington, who hopes to beat Rep. Joseph L. Fisher in the 10th District of Virginia, and Malcolm McKnight, who is apposing rep. Clarence Long in Maryland's Second District, were among those attending. Last night, two area incumbents, Rep. Newton I. Steers of Maryland and Rep. Paul Trible of Virginia, talked about how they captured open seats - where there is no incumbent - tow years ago.

Trible's wife, Rosemary, also offered the women tips on how to deal with reporters. "Don't be intimidated by the press," she advised, "but don't be overly aggressive either. Get to know them on a first-name basis, form a personal relationship, let them know they're an important part of your campaign."

Jeanne Hyde, wife of Rep. Henry Hyde of Illinois, supplied another reason to "find out who's assigned to your campaign." She learned the hard way not to assume that "all those people milling around are supporters" after she read in the newspaper remarks she thought she was telling in confidence to a friend.

Mrs. Brown agreed that "you might be misquoted," but worse than that, she said, and more likely, is that "you won't be quoted at all."

Proposition 13 came up, but Finkelstein said the California referendum result shouldn't be interpreted as "evidence people want services cut. They want taxes cut," he said, and voters aren't sophisticated enough to understand that one leads to the other.

The cynical Finkelstein cited the 1965 New York mayoral campaign of John Lindsay, who he said was elected because "he promised to cut the budget in half and double the services. Four years later, when it was apparent that he had doubled the budget and cut services; Lindsay said, 'Yes, I made a mistake,' but he was re-elected when he promised to help end the Vietnam War."

The husky, sandy-haired Del Giorno hopes to upset Rep. James M. Hanley, a seven-term Democrat with the help of information he and his wife wrote in the blue notebooks provided at the workshop. In large red ink, Del Giorno underlined six items he hopes will be keys to a victory: research, strategy, tactics and timing, plan development, budget and financing.

Del Giorno figures he'll need to raise $140,000 to wage the kind of media-heavy campaign needed to knock off an incumbent.

Meanwhile, his wife is working on an image problem. "Used car salesman," she admits, doesn't instill confidence in voters' minds. So at the workshop, her husband's occupationis listed as "auto sales and service."