Creating a vamp from scratch?

Create a new household. When you click to add a sim, select Occults (it's the swirl) and then the occult type you want. Then delete the random sim the game created when you created the household.

The other way is to make friends with a vampire and ask to be turned. Turning takes about a day-and-a-half and is quite uncomfortable. Try to make sure you're out of the sun and away from garlic when the process finishes. If you do get stuck in the sun, I recommend using your first vampire point for batform, so you can fly inside quickly.