Visit any inn and ask the innkeeper if they have heard any rumors. They will tell you that a wizard in Morthal named Falion studies vampires. Travel to Morthal and meet with him. He will ask you to bring him a Black Soul Gem (you can buy one from him for 280 septims). To fill the black soul gem you must cast Soul Trap or use a weapon enchanted with soul trap on a human and then kill them to get their soul. Once filled, take it to Falion and he will instruct you to meet him at a summoning circle just outside of town at dawn. Once there, he will preform a short ceremony and your vampirism will be cured.Note: You can become a vampire again even after the ritual rids you of the disease.You can also cure your Vampirism by becoming a werewolf within the Companions faction. However, it is rumored that you cannot return to Vampirism afterwards.