Skyrim Dawnguard Walkthrough

To start your quest dawnguard must have level 10, after you install Dawnguard addon, please speak with one of the big city like Whiterun guard Guard to begin Dawnguard questline.
Start the quest Dawnguard open your quest log

Dawnguard
I’ve learned of a group of vampire hunters known as the Dawnguard. It seems a They are looking for new recruits.

Objectives:Speaking with chairman Dawnguard.

Another way to start this quest is to talk to Durak WHO will come to you, Sometimes after you are attacked by some vampires.