I am a vampire and I want to be human again?

Go to any chapel, talk to a healer/priest (or Vincent if you are in the Dark Brotherhood). The healer/priest will say something lame, then point you to Ramius Polus in the Arcane University. He's mainly in the main tower room during normal hours. He'll point you to the Count in Skingrad, who will then point you to a Which that lives out in a cabin. Talk to her, (entering her cabin will show the "red" door icon, don't worry about it, just enter). She'll talk, then she'll send you on a quest to 5 Grand Soul Gems (empty) before she'll tell you how to make the cure. She'll then make you get 6 garlic cloves, 2 bloodgrass shoots, 5 nightshade leaves, Argonian blood, and the ashes of a powerful Vampire. The latter two she will supply you with the object/information/map marker you will need. Then just follow the rest of the quest through, and you will have a cure Vampirism.

If you come with the supplied items before actually getting the quests from her, take note you will have to completely end talks with her and then start again to make her take the objects from you.