You have to do the Tamer quest chain that starts in either SW or Org and work your way up to the Northrend ones. The trainer in SW is near the little house between the Dwarven district Bank and the Cata Portals and the Org one is up on the ledge by the flight master.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."