You’ll be pointed towards a mage in Morthal called Falion. To get started you’ll need to speak to any bartender and ask for rumours until you’re given the quest “Rising at Dawn”. So if you’re Stage 3 or 4, get feed first before you start. In Stage 3 NPCs will not usually interact with you and at Stage 4 you are so far gone that citizens will attack you on sight. It is inadvisable to attempt curing vampirism if you are Stage 3 or 4. You progress through the stages every 24 hours if you don’t feed on a sleeping NPC.
The first thing to know about being a vampire is that there are four stages and each stage brings you more benefits but also more detriments.