The origins of It are left a bit of a mystery in both the book and the movie, but the book at least touches on how It came to be in Derry by having the kids enact a Native American ritual that Ben read about at the library. They fill their clubhouse with smoke, hoping to receive a vision that will help them learn more about It. While all of the Losers participate, only Mike and Richie wind up being transported millions of years into the past, where they witness It crash into Earth from space like a meteor.
In the film, however, the adult Mike visits a Native tribe living outside of Derry, who fought It once many years ago. They give him herbs with induce hallucinations, which is how Mike learns about the extraterrestrial origins of It. Mike then slips a lower dose of the same herbs into Bill's water glass, unbeknownst to Bill, in a last-ditch effort to get Bill to help him. Once Bill also sees the arrival of Pennywise, he changes his mind about leaving Derry, and agrees to help Mike persuade the others to stay and fight.
In a subplot from the book which is completely omitted from the movie, the adult Losers are horrified to learn that It is not only female, but pregnant. They discover over a hundred eggs, which Ben Hanscom stomps on, determined to squash every single one. In the film, although It most often takes a male form, It is never given a specific gender, and there's never any talk of eggs.
Also, the It of the novel takes a number of different forms throughout the narrative, including a werewolf, a giant bird, a massive crawling eye, and its true form, a giant spider. But although the film version of It can indeed shapeshift, and does assume some of its forms from the novel — most disturbingly, the decaying head of Stan Uris — it spends most of its time in the recognisable visage of Pennywise the Clown. It does spend some time as a version of a spider during the final battle, but it always wears Pennywise's face, altering some of the nightmarish imagery from the book.