It depends, for the new rules (Vampire: the Requiem), you'd need the basic rules book "The World of Darkness - storytelling system rulebook" and the specific Vampire rulebook "Vampire: the Requiem - a modern gothic storytelling game". There are of course expansions and also the books in the Werewolf and Mage serieses.
For the old rules (Vampire: the Masquerade), I don't really know whether they're still published, but they migh still be available on e-bay or so.
And here is a link to
whitewolf's site where you might find more information.