So you want to start leveling some characters and you find y ourself wondering what to pick, theres 10 to chose from!
Well OK, so no Death Knights till you get to 55 on a character but you get the picture.
What I suggest is 4 characters to level +1-2 'bank alts'.
1 Tank class, 1 DPS class, 1 healer Class, 1 Hunter. This will let you try out the various roles in the game and if yu dont like them you can do something else. For example if you pick a Druid as you 'try a healer' and decide you hate it you can always try it as a tank or dps.
Oh why a hunter specifically? their great at solo leveling and hunting/gathering to support other characters. Everyone should have a hunter. :D
Warrior, Death Knight
Rogue, Hunter, Warlock, Mage
So what to do if you want to have a death knight? Just be patient. Level your other characters and when you get one to 55 you can roll a Death Knight. But realize she will end up 58 when you finish the starting zone so she will be 3 levels above your other toons. You might want to wait a bit just so she isnt too far ahead.