SCREEN: 'THE LEGEND OF BILLIE JEAN' OPENS

''THE LEGEND OF BILLIE JEAN,'' which opens today at the Gemini and other theaters, is another unfortunate film about a nobody who gets to be a somebody, 80's-style. This entails appearing on television, getting one's name on Frisbees and T-shirts and ''I H So-and-So'' bumper stickers, being mentioned on talk shows, and winning the spontaneous applause of the crowd. Here, as in ''Turk 182!,'' the main character need hardly do anything in order to win this adulation.

The film gets off to a simple enough start, which makes these head-swelling developments all the more regrettable. Billie Jean Davy, a gorgeous tawny blonde played with that ''Flashdance'' brand of flirty innocence by Helen Slater (''Supergirl''), lives in a trailer camp in Corpus Christi, Tex., and is the kind of innocent, barefoot belle who can't help but attract the wrong kind of attention. Bullies love to chase her and her younger brother, who is named Binx (played by Christian Slater, who is no relation to the star).

When they inflict 608ドル worth of damage to Binx's motorcycle, Billie Jean insists on justice. She goes to see the father of the boy who started the trouble. She demands the money, and he makes some lascivious demands of his own. Fortunately, Billie Jean has a mean right knee, which she has at least three opportunities to use during the course of the story. Less fortunately, Binx accidentally happens onto a gun and fires it, turning himself and Billie Jean into modern-day outlaws.

It is at this point that everything goes wrong, since Billie Jean, once she's on the lam, doesn't even attempt to keep a low profile. She doesn't have to, now that she's an instant star. Before she even knows it, some teen-agers she meets in a store are asking for her autograph and offering to buy candy for her and her accomplices, who have taken to calling themselves ''the Billie Jean Gang.'' The supplies are for them. Billie Jean herself isn't the type to eat candy.

The film, which was directed by Matthew Robbins (''Corvette Summer''), takes an even wilder turn when Billie Jean watches Jean Seberg play Joan of Arc in an old movie on television one day. She thinks about this briefly, then cuts off all her hair and develops serious delusions of grandeur. Mr. Robbins, who gives the early part of the film a warm glow and a brisk momentum, handles the Joan of Arc angle with no noticeable irony whatsoever. Nor does he develop any when Billie Jean's look supposedly becomes even more popular with teen-agers than Madonna's. The film contains a closing credit for a ''Billie Jean Cut,'' by a Beverly Hills hairdresser.