While Death Knights start at level 55 they will still have to train any profession skills from the very beginning. The only exception is first aid, where they start at a skill level of 250. Every other skill needs to be trained beginning at 0. The only advantage is that Death Knight have access to every flight point in the Eastern Kingdoms and in Kalimdor and start with an epic ground mount, which makes leveling up those skills somewhat easier.

It is, of course, a matter of taste which skills are best suited for a Death Knight. As a plate wearing class blacksmithing is an obvious choice. As blacksmiths get the ability to add sockets to their gear at level 80 this choice isn't a bad one later on, although skilling up blacksmithing is rather expensive and exhausting.

The complementary skill to blacksmithing is mining, as it is needed to supply the blacksmith with material. If one already possesses a character with mining you can select jewelcrafting as your second profession, giving you access to some high quality items. Especially the jewelcrafting gems that can be created later on make this profession on of the most effective. The only downside of this combination is the high usage of ores, so this is only advisable if you have a character with mining already.