In order to be able to start a demon hunter character, you must have a level 70 or higher character on any server (it has to be on the same Battle.net account and realm). When the character is first created, you will must do a series of quests mostly on the world fragment Mardum, the Shattered Abyss. By doing these quests, you will learn how to bend the power of the Demon Hunter to your own will, and after you have mastered that, you are then capable of going to any other area.