Michael Jackson, now 20, an elongated version of the cute, spindly youngster who fingerpopped into teen hearts 10 years ago, knows what he wants for the next 10.

"To do everything I feel I should do," says Jackson, his trademark airy pitch unchanged. His dark eyes are direct, making no excuses for that goal. "Really, more music, films, everything. I want to go all the way." His smile grows into grin, testing the boundaries of a cresent fact that, up close, is small and sandpapery. His answers are the quick, flippant retorts of any 20-year-old, their tone mixed with the blase worldliness of someone who has spent half his life in the limelight.

Dressed in brown slacks and print shirt with a gray cavalry hat perched on a wayward Afro, Jackson leans back onto the bedpost. His brother, Marlon, 22, one of the original five who started out as the Jackson Five in 1969 and renamed themselves the Jacksons three year ago, joins the conversation. Three hours before the group will bring 20,000-plus fans to their feet, on a night when a reported 5,000 were turned away, Michael and Marlon are totally relaxed. In fact, they are cutting up like the Smothers Brothers.

"What! There aren't any girls downstairs," mocks Marlon, camouflaged behind sunglasses and a worn cream-colored jogging suit. From the corner, one of the managers announces that the lobby of the Sheraton Lanham Motor Inn was packed earlier in the day. "Well I guess they'd expect us to be at the Regency, or the 'Gate," says Michael (who is given to abbreviations: 'Gate for Watergate Hotel, 'tics for politics, 'Town for Motown Records, their first label).

Ten years ago, the Jacksons were all terribly green, painfully shy, leaving all the declarations to their father, Joseph Jackson. What they offered to the music scene were five blemishfree faces, heart-throbbing in their close-cut hair, chino pants and matching sports jackets. Out of Gary, Ind., they marketed bubble-gum soul, which brought them adulation and riches. But, even then, when they spoke, they were coy: their sound, explained one back then, "is a secret; too many people might find out and start doing it."

Now, with their youth no longer a salable part of the act (Randy, the youngest, is 16), the Jacksons have to compete with established male acts like the O'Jays and Commodores. "We have to strive to set trends, instead of following them," says Michael. They are succeeding. Their latest album, "Destiny" is certified platinum and has birthed two top 10 singles.

Yet 10 years has produced some changes. Chinos have been replaced by coordinated gold lame stitched into a medieval-futuristic combination. The screams have cultivated layers, from those whose years are marked by Jacksons' pin-ups and from a younger generation that can't be called bubble-gum, though that's its age and beat. In personnel, there have been alterations - one brother, Randy, substituting for another, Jermaine, who remained with Motown when the rest switched to CBS Epic Records.

But what has remained constant is the dominance of Michael Jackson. His career has gone further, expanding to movies with the role of the Scarecrow in "The Wiz," and joining the gossip mystique, escorting Tatum O'Neal. On stage Michael's dancing is an impeccable sample of disco and acrobatics, stylishly flamboyant and patterned. The packaging has killed the spontaneity but, nevertheless, three women were carried over the rails in dead faints during Michael's solos Saturday night.

"In Charlotte, it was a little unreal because they carried the girls out in stretchers across the stage," says Michael. That was Friday night, and a day later, he sounded slightly stunned. Marlon explains that they never get used to the screams, and Michael elaborates, "it honestly feels fresh each time."

In the hotel room the two brothers are explaining what tours are like, insisting the Jackson public camara-derie carries over to travel and record sessions. In the fourth week of the two-month tour that ended over the weekend, the Jacksons switched from airplanes to a caravan of station wagons and mobile homes. "I prefer the bus. Flying is fine in good weather," says Michael, who watches movies during the long drives and then rushes out to catch the zoo in each major city. "Hey he feels at home, looking at his relatives," says Marlon.

"Well, that means you, right? Sure I like you," says Michael.

Looking back, the Jacksons agree that the response they triggered in fans was the most fulfilling aspect of the last decade. "The fact that we have sold 60 million records and brought joy to so many people. I like that for the happiness, not for the money," says Michael. "And playing for the Queen of England, twice; what an honor. I had a fantasy of kings and queens and there she was in her magic box, with her crown and jewels." Marlon interrupts. "It's everyone we have touched, and selling out certain places, like the Astrodome and breaking the Beatles record in Liverpool."

Success has brought certain luxuries. Marlon is buying a house in the San Fernando Valley, equipped with a tennis court, so he can live his fantasy of being a tennis pro. Waiting for Michael at the family's sprawling compound in Encino, Calif., are several movie offers.

"First there's the movie of 'Chorus Line,' but I haven't seen the script yet. Then someone is doing a script on the Bill Robinson story, then a movie about summer stock, about what people do to make it, the pain of success," says Michael. Is he attracted to stories of struggle because his career has been free of scars? Michael immediately responds yes. Marion again interrupts. "In the early days, before we went national, we did the seven shows a night, traveling in a Volkswagen. That wasn't fun." Corrects Michael, "It hasn't been bad, but we didn't come out of the blue."

But what about the internal jealousies, the rifts that must come? "We like one another and think alike, so there are few problems. And we tell this one," Marlon says, looking at Michael, whose attention has been caught by the television, "We tell him the most important thing is the last name." Michael scowls. And Marlon amends, "I was only kidding." CAPTION: Picture, Michael left, and Marlon Jackson; by John McDonnell - The Washington Post.