Report to Williden Marshal at Marshal's Stand in Un'Goro Crater. Optionally, grab a flight there from Doras in Orgrimmar.
All able bodied members of the Horde are hereby ordered to report Williden Marshal at Marshal's Stand. You are to provide aid to the Marshal Expeditions' personnel as they uncover the secrets of Un'Goro Crater. The Horde may be able to find a use for the unique beasts and crystals populating the crater.
You can reach Un'Goro Crater by taking the road from western Tanaris down into the crater and toward Marshal's Stand. Or, you can get a flight there from Doras in Orgrimmar.