Visit Falion in Morthal
Ask him to cure your vampirism
Bring a filled black soul gem
Fill the black soul gem with a soul
Return to Falion at dawn
Stand in the ritual circle outside Morthal
Let Falion perform the cure
If you are a vampire lord, cure it the same way
If you are a werewolf, use the Companions to become a werewolf instead and it will remove vampirism
If you have not progressed too far, drink a Cure Disease potion or pray at a shrine before becoming a full vampire