Question: how to do dawnguard quests?
Answer: The quest can be given by either a Hold Guard or an Orc named Durak after entering a hold capital at level 10 or after. It can also be given before level 10 by talking to Agmaer in Dayspring Canyon. They'll ask the Dragonborn about joining the Dawnguard, an order of vampire hunters based in Fort Dawnguard, near Riften.