Go to any inn and ask about rumors until you hear about Falion in Morthal
Travel to Morthal and speak to Falion
Ask Falion to cure your vampirism
Buy a filled Black Soul Gem from Falion or any merchant
Fill the Black Soul Gem by trapping a humanoid soul with Soul Trap
Return to Falion with the filled Black Soul Gem
Meet Falion at the summoning circle outside Morthal at dawn
Let Falion perform the cure ritual
If you are a Vampire Lord, cure vampirism through Falion the same way
If you are infected with Sanguinare Vampiris, drink a Cure Disease potion or pray at a shrine before it turns into full vampirism