Go to a Vigilant of Stendarr, innkeeper, or court wizard and ask about curing vampirism
Start the quest to cure vampirism
Fill a Black Soul Gem
Bring the filled Black Soul Gem to Falion in Morthal
Meet Falion at dawn at the marked ritual site
Complete the ritual to cure vampirism
If you are a Vampire Lord, cure that form the same way
If you are a werewolf, cure lycanthropy first before curing vampirism
If you are only infected with Sanguinare Vampiris, drink a Cure Disease potion or pray at a shrine before it turns into full vampirism