Where do Death knights start?

Death knights start at level 55, in a floating necropolis named Acherus: The Ebon Hold over the eastern plaguelands. This necropolis is 'nuetral' ground, so no pvp can take place there - it is also instanced, and only death knight characters will be able to get into acherus.. Within the necropolis, new deathknights follow a series of quests that teach them how to use each of the abilities the character starts with, as well as introduce them to the lore surrounding the character and class.

Death knights start with a full set of green gear, minus trinkets, a sigil, and weapons. The second quest you take as a death knight rewards you with a blue-quality weapon, and as you do more quests within the deathknight starting chain you are rewarded with better gear. By the time you finish this quest chain - around level 58 - you have a full set of blue gear including trinkets and a high quality weapon. This gear is about equivalent to level 62~ outland gear.