Instructions
Contract the disease "porphyric hemophilia" to become a vampire. If you're in the Dark Brotherhood and you've completed the quest "The Assassinated Man," then you just need to speak with Vicente Valtieri in the Dark Brotherhood Sanctuary and select the topic "Dark Gift." If you agree to become a vampire, the next time you sleep in the sanctuary you'll awaken as a vampire. Otherwise you must visit a vampire dungeon and fight vampires until you catch the disease. Two good locations are "Fanacas," Ayleid ruins north of Cheydinhal, and "Fort Vlastarus," an abandoned fortress east of Skingrad.
Wait for 72 in-game hours for the disease to take full effect. You can continue playing the game or simply wait or sleep for that period.
Sleep in a bed or bedroll after the 72 hours are up, and you'll wake up as a full vampire. The longer you go without "feeding," or drinking another character's blood, the stronger the disease becomes, and all its effects become more pronounced. The effects are both good and bad: you gain magic spells, many of your skills and attributes receive bonuses, and you have a resistance to normal weapons. On th other hand you also have a weakness to fire damage, and, barring the first 24 hours after each "feeding," you take damage in the sunlight.
Locate a sleeping person to feed on. Obviously this is easiest during the night. You can look for beggars sleeping outdoors in cities or you can try breaking into homes or rooms at an inn. If you join the mages' guild or the fighters' guild, you have plenty of easy targets in those buildings. Just keep in mind that if anyone spots you picking a lock or entering a place you shouldn't be, they'll either demand that you leave or call the guards.
Approach the sleeping person carefully while "stealthed," and make sure no one is around. Remember that the faster you move, the more likely the person is to hear you and wake up. The boots you wear matter as well, so don't wear armored boots. Any rings or enchanted clothing that increase your stealth will help. If someone catches you feeding on a person, they'll call the guards, so take a good look around before starting.
Press the "X" button on your game pad when the cross hairs in the middle of your screen change to indicate you can interact with the person. A menu appears. Choose either to wake the person and speak to him or to feed on him. Choose the latter, and your character will drink the blood of the resting person.