Fieldplate. With complete study of Smithing, Fullplate. Plate limb armor.
Balance type: Primarily balance, with some few combination abilities using balance and equilibrium. AB BRUTALITY DOUBLESLASH and AB BRUTALITY ENGAGE, for example.
Brutality - This is your primary skillset. It is most important for both PvE and PvP. It focuses on the usage of bladework, primarily. The existence of a deathknight is in no way kind, and their studies reflect that. Your accuracy, speed and damage are all increased by your knowledge of Brutality.
Necromancy - The power to conquer death, and exist beyond it. Necromancy is your support skillset. There are some few offensive abilities as well, but for the most part this skillset is utilitarian in nature, and survival oriented.
Smithing - Smithing is a tradeskill ability that, at present, provides little combat value. Upon completing it, Deathknights obtain the ability to wear fullplate, the strongest armor in the game. If you've got credits to spare, it's not a bad investment. Otherwise, this should be far down the list of things you want to invest in. Repairing also comes standard with the skillset.
Speed and afflictions. The deathknight's primary kill method is Vivisection (AB NECROMANCY VIVISECT), achieved by having the arms and legs of an opponent simultaneously broken to a level 1 (mending cured) break. With targeted and displaced doubleslashes, and intelligent affliction delivery, this is the most effective way to kill as a deathknight, although for a beginner it is not suggested, as it is a bit involved. Also effective in group combat with damage, with the ability to quench a blade temporarily to a specific target's soul, giving a chance to deliver extra damage with every swing. In this instance, a higher strength is more important than a faster balance.