How did Carrie get her powers?

It's never explicitly said how or why Carrie has the powers that she does; the best explanation is actually not even from that book, but from Firestarter instead (since King's works often criss-cross and connect and the two books were written in reasonable proximity to each other, it's not unreasonable to compare the two). read more

The end of the novel had Carrie engage in a telepathic conversation with Sue, after Sue felt Carrie drawing her in with her telepathy and ended up getting "mindraped" when she invited Carrie to read her thoughts to prove her innocence; this was a painful experience for Sue. read more

While the film never explains the origin of Carrie's powers as the source novel does, it appears that it was genetic in origin. It manifested itself when she hit puberty and it seems, according to the films narrative, that it intensified when she was upset, angry or stressed. read more