How many people have done a starring role as their debut?

I know that my plugging of this film at every opportunity has become almost a joke, but it actually fits the OP to a T - John David Carson in Pretty Maids All in a Row.

No acting experience whatsoever, except for stage productions at Los Angeles Valley College. 19 years old - and his first film role has him starring opposite Rock Hudson, Angie Dickinson and Telly Savalas. Those actors obviously got top billing, and Carson was billed as “introducing John David Carson,” but his character, Ponce de Leon, was the true protagonist of the film.

(That movie was also Carson’s only starring role. His career didn’t exactly take off, and he drank and gambled all of his money away. He’s supposedly now homeless.)

Not entirely fair, as she had been a Broadway star in The Sound of Music and My Fair Lady. At the time it was FAR more than equal to later Broadway stars, especially since both Original Cast Recordings were MASSIVE LP hits. It’s like claiming Elvis only became a star because of the success of Jailhouse Rock.

Natalie Portman in The Assassin.

The Professional. Alternatively, Leon.

Christopher Atkins in The Blue Lagoon.

Thomas Turgoose, star of This is England, not only hadn’t acted professionally before but had recently been rejected for a minor role in his school play. He won a British Independent Film Award and was nominated for a few others for his performance.

Mark Frechette and Daria Halprin were the two leads in Zabriskie Point; neither had been in a film before.

Sorta on the subject, sorta off.
Duane Johnson aka The Rock, was paid $5 million for his first starring role in a movie, The Mummy Scorpion King. (the second one).