This question has been asked many times already due to people being too lazy to read previous topics.
Basically have ONE character at least 55, then bam, you unlocked the Death Knight class.
Create Death Knight like any other character you would create on ANY realm, limit ONE Death Knight per server.
I don't completely blame Tehnomage for asking that question in THIS thread.
Horrible thread labeling makes people ask the same question over and over again, i.e. like the naming on this thread.
Can it get anymore broader than "DK"?