Buffy The Vampire Slayer is one of those cases where the movie is far worse than the TV series. The major thing the movie does is introduce us to Buffy as the Slayer, whose destiny is to fight and slay vampires.
Kristy Swanson plays Buffy, a blonde high school cheerleader with little on her mind besides graduating, going off to Europe, marrying Christian Slater and dying. Hardly the material you would expect from "the chosen one." In the words of Merrick (Donald Sutherland), who has to train Buffy to realise her full potential, there couldn't have been a more vacuous choice to play the Slayer.
Reluctantly, Buffy takes on the responsibility of being the Slayer after a graveyard camp out with Merrick where they are attacked by vampires. The movie culminates when the entire valley Buffy lives in is attacked by vampires and Buffy has to fight and defeat the Vampire big boss Lothos (Rutger Hauer) to save herself and the town.
As I say above, the best thing about this movie is that it gives an introduction to the TV series, but the TV series does this in a few seconds, so by all accounts, this movie has no good reason to exist. Further, the TV series is far more interesting due to the presence of Buffy's friends, and the portrayal of Buffy is far less vacuous. (I actually enjoy the TV series.)
My advice is that it's worth renting just to get a perspective on the TV series, particularly if you're a fan of it.