Challenge Tenwu of the Red Smoke, and then defeat him in combat at Shado-Pan Garrison in Townlong Steppes.
So... you've been challenged by the infamous Tenwu of the Red Smoke. That's what he calls himself, at least. He's a good fighter, and a good rogue, but it's a wonder he manages to sneak up on anyone with how loudly he brags about himself. Anyways, I think you should answer the challenge. Maybe it'll give him a touch of humility. Maybe not. Either way, if you win, he'll probably be willing to go with you on your missions. And I get some entertainment.