It's so short, in fact, that we can give you most of the info right here. You'll be able to create a Death Knight on any realm on which you already have an level 55 or higher character. Eventually, you'll be able to create one on a different server, but for now the only way to get a Death Knight to a totally new realm is to create it on your current realm and then do a paid transfer.
Don't worry -- there's good news too! If your level 55 is Horde, you'll be able to start up an Alliance Death Knight and vice versa, so long as you're not playing on a PvP realm. You're only allowed one Death Knight per realm though.
[Via WoW Insider]