In the Blasted Lands, go south to the Dark Portal and take it to Hellfire Peninsula; if lv. 58, the quests available will lead you to the Thrallmar encampment. You will not be able to pass through the dark portal into Outland unless you are level 58 or above.
NOTE: There is now a portal to the Blasted Lands by the Mage Trainer in Orgrimmar
From Falcon Watch in Hellfire Peninsula, follow the path west past the Cenarion Post and Thornfang Hill to reach Zangarmarsh. Continue following the path to reach Cenarion Refuge.
From the Cenarion Refuge in Zangarmarsh, follow the path south, past Umbrafen Village into Terokkar Forest. Follow the path south until it splits; take the eastern one to get to Cenarion Thicket, or continue south to reach Shattrath City.
From Zabra'jin in Zangarmarsh, follow the path curving from west to south, down to Nagrand. Stay on the path SE/E until you reach the Horde encampment of Garadar.
(Recommended) From the Shattrath City in Terokkar, follow the path south into the Bone Wastes. Take the path that exists west from this dead land, and follow it northwest into Nagrand.
Blade's Edge Mountains
From the Cenarion Refuge in Zangarmarsh, using the path, go north, northwest/west, then north past Telredor. You will eventually reach Blades' Run in Blade's Edge. Continue north into Thunderlord Stronghold.