Quick mini-guide here on how to become a vampire or werewolf in Skyrim.
Vampires:First a bit of an intro. Being a vampire in this game is similar to being a vampire in Oblivion, however there definitely are some key differences.
Basically, when you're a vampire you gain some serious benefits, at the cost of being weak to sunlight. The longer you go without feeding (drinking blood from a sleeping victim), the more powerful you become, but you also are increasingly weak to fire and sunlight. In addition if you go more than 72 hours without feeding (stage 4 vampire) every NPC turns hostile.
Without further ado, here's the skills and benefits from being a vampire (copy pasted from the wiki)
Werewolves I don't know as much about being a werewolf, however here's what the wiki has to say. I plan on making a Warrior werewolf character next, so I'll update this when I know more personally.
Right, onto the srs bsns of it. In order to contract the disease to become a vampire, you need to be attacked by a vampire. The difficult part is finding a vampire in the first place. My main character is a vampire, and here's where I contracted the disease.
Once you have the disease, you have 3 days to cure it before you become a full fledged vampire. There are plenty of other places to find vampires and become one, however I have personally confirmed the prior method (I'm a vampire myself). If you guys have any other locations, post them and I'll try and confirm them.
The second method contains a SPOILER FOR THE COMPANION QUESTLINE OPEN AT YOUR OWN RISK.
I don't know as much about being a werewolf, however here's what the wiki has to say. I plan on making a Warrior werewolf character next, so I'll update this when I know more personally.