Visit any innkeeper and ask about rumors until you hear about Falion in Morthal
Go to Morthal and speak with Falion
Buy a filled black soul gem from Falion or any merchant
Fill the black soul gem if it is empty
Return to Falion with the filled black soul gem
Meet Falion at the dawn ritual site outside Morthal
Let Falion perform the cure ritual
If you are a Vampire Lord, use the Vampire Lord skill tree to revert to human form first if needed
If you are fully turned into a vampire, act before the disease progresses too far if you want the easiest cure
If you are a werewolf, use the Companions questline cure method instead of Falion’s ritual