There are no prerequisites to obtain the Keldeo. So you can just go to GameStop (or EBGames, which I so miss here in the USA) and they'll give you Keldeo.
However, in order to teach Keldeo the move Secret Sword, you will need to catch and have in yourr party Cobalion, Terrakion, and Virizion.
Also, just a tip you might find useful. You don't need to beat the Elite 4 in order to catch Cobalion, Terrakion, or Virizion.