Right...basic overview:
Shannen plays a girl called Heather and Jennifer Blanc plays a character called Zanne.
The movie begins with a sort of talent competition between the Heather and a girl called Suzanne when they were younger (different actresses, obviously lol), with Heather beating Suzanne because Suzanne sorta freezes on stage.
So yeah...it skips to when they are older and Heather is popular and in a band and a sorority and has a boyfriend etc. There's a new girl called Zanne and Heather is accomodating to her at first and helps Zanne to fit in etc and pulls a few strings to get her into the same sorority.
Anyways, Zanne is clearly jealous of Heather and works things so she eventually gets Heather's place in the band and steals Heather's boyfriend. Heather gets left out and all of her friends turn against her because Zanne manipulates them so they think she's a liar etc.
Heather starts sorta investigating who Zanne really is and finds out that Zanne is really Suzanne from the talent competition from when they were younger. It turns out Zanne got accused of hurting the winner in another competition so she could win (this was after the competition Heather won).
Suzanne ended up being in and out of psychiatric hospitals and eventually left home, changed her name and pretty much set out to ruin Heather's life and be a winner I guess...re-write history in some weird way. She wanted what Heather had.
So yeah...at the end Zanne gets caught and Heather performs with her band in a competition because Zanne ends up freezing on stage again.
I'd go into more detail but that could wreck the movie if you get to see it...I just hope that makes sense lol.