NOTE : There have been some comments getting out of hand. I am deleting any rude or personally insulting comments in this thread. Keep it clean.
So in reading my blog referrers, it seems becoming a vampire is a problem many people have, so I decided to write a little vampire guide. This guide contains some mild spoilers. be warned.
Becoming a vampire is really easy. There are two major paths
1) Fight a vampire.
2) Recieve the “dark gift”
There is not a way to make yourself a vampire via spells, a ritual, or anything else like that.
Two notes : 1 )You cannot become a vampire in GOD mode, if you are cheating, it wont work.
2) If you cure yourself, you cannot become a vampire a second time. You are permanently cured.
The Dark gift is available as a Dark Brotherhood quest reward. To get this, first join the Dark Brotherhood. You may be able to join the brotherhood by going straight to their hideout and sanctuary in Cheydinhal, but the standard way is to kill someone. This is slightly confusing, because you can't kill just anyone. You need to kill someone who is not attacking you, and who is a member of at least one evil faction. The easiest ones in my opinion are the Deadra shrine worshippers. Complete a Deadra shrine quest, then kill the worshippers (do the quest first, if you kill first, you can't do the quest). Then go sleep somewhere safe (an inn or something), that will lead you to the Dark Brotherhood. Do a few of the quests, and bingo, Vincente will make you a vampire.(You can choose not to accept the gift as well)
The other way to become a vampire is just to fight vampires. There are MANY vampire covens around, and many side quests that lead you directly to Vampires. A few of the quests and where to start them are listed below. When you fight a vampire, you may contract Porphoric Hemophilia. At this point you are not a vampire. If you drink a cure disease potion, or visit a cure shrine, you will not become a vampire. However, if you wait 3 days, then sleep. You will become a vampire. Fighting vampires is easiest with fire spells or weapons, and some detect life spells, because vampires can become invisible. For best effect, let the vampire hit you without fighting back too much, so that you have more chances to contract the disease.
Being a vampire has perks, and some downsides. There are 4 levels of vampirism. At the first level, its pretty much all perk. You are stonger, sneakier, more powerful with magic, and have a night vision & detect life power. The only downside is an increased weakness to fire.
Each 24 hour period you go without feeding, you become more of a vampire. You get more powerful, and gain additional spells (like invisibility). However, at levels 2-4 you take damage from the sun. At level 2, the sun damage is not too bad, a mage can run around and heal faster than the sun hurts. However, at levels 3 and 4, the damage is quite heavy, and you will need to stay indoors. Also, at level 2-4 you cannot fast travel during the day. The further you get down the vampire path, the less people will like you. If you have high personality or speechcraft, this isn't a problem. You only increase levels when you wait or sleep, so you can go for many days at any given level as long as you don't wait or sleep. If you need some time to pass, just run around, or fast travel to another city, and do a step or two of a side quest. I prefer to stay at level for the most part, so that I can fast travel during the day. But if I am going to fight in a hard dungeon, then I wait for 3 or 4 days to become stronger. If you are taking sun damage, just go inside and wait until night. This serves 2 purposes. 1) you wont be taking sun damage anymore, and 2) people are now asleep so you can feed on them.
Feeding : Its easy to feed, just walk up to someone that is sleeping, and try to talk to them. You will get a choice between talking and feeding. Feed, and you are back to level 1! There are many easy targets for this, any of the guild halls, the blades, sleeping bums, or random people's houses.
Curing Vampirism : I have not done this (because I like being a vampire), but there are several ways to start the cure vampirism cure. You can talk to the count of skingraad, Vincente in the Dark Brotherhood, or your contact in the Mages Guild.
Quests to meet vampires, note these may be considered mild spoilers.
Ulterior Motives - Mages guild quest, Arcane University, Imperial City (Vampire Hunting Quest)
The Order of Virtuous Blood - Temple District, Imperial City (Vampire Hunting Quest)
Lost Histories - Thieves Guild quest, eventually vampire related
Origin of the Grey Prince - Side Quest, Arena, Imperial City
A Brotherhood Betrayed - Side Quest, Bruma - THIS IS NOT A VAMPIRE QUEST, but it seems like it.