There's raiding or instances. But to do these, you'd need a group. And long-term, you'd need a guild. You can start off in a small or growing guild and do Karazhan, only requires 10 raiders. Gruul onwards will require 25raiders so by then you'd need to be in a guild of at least 30people.

As a warrior, you'd be more in demand as a prot if you do instances or join a raiding guild. So you may want to give that a thought; go prot and do instances or raid. You don't need to grind rep to kill bosses and clear raids.