There are only three ways to become a vampire. The most easy way is to forget to cure yourself of Porphic Hemophilea within three days. It is likewise to become a vampire while in the Dark Brotherhood.

1:
You will not take any sun damage. You will start having an immunity to diseases. You can start feeding on sleeping bodies.
2:
You will take 1 sun damage.
3. You will take more sun damage.
4: You'll take a moderate amount of sun damage.

To return your vampirism stage to one, you must feed on a sleeping body.

Only one is added by each stage of Vampirism.

There are only two ways to cure Vampirism:

Drink a vampirism cure potion after completing the quest Vampire Cure.
If Vampire Lair is installed, you can use the pillar at Deepscorn Hollow.

It is possible to do this yourself.