Follow the western road from Kargal's Far Watch Outpost. At the T intersection, turn left and follow the road south. Find Sergra Darkthorn at the crossing of roads within the Crossroads.
Take this signed recruitment letter to the Crossroads.
Sergra Darkthorn, curse her hide, is in charge there. She's a nasty shaman who will talk your head in circles before she lets you do anything useful. Thrall is convinced she's something special, but I'm not so sure.
You'll just have to decide for yourself, pup...