How to start playing world of warcraft cata?

this is my situation...i've mostly played wow on privet servers and this is my first time playing on retail...i've had WOTLK installed and just today i've bought cata and installed it but for some reason i can't add wow to my battle net acc and the key i've gotten from my retail copy won't work and o i can't play the game only the trail account...can some1 explain to me step by step what to do...how to get starter...the blizzard support doesn't help much for new players joining retail..