Past level 80, the quickest way to is take the portal from Stormwind to Uldum and simply fly to Tanaris. Otherwise, take the Tram to Ironforge then fly (or walk) to Menethil harbor. From there, take the boat to Theramore and swim through Thousand Needles (south of Dustwallow Marsh) and right into Tanaris.
Or take the ship from Stormwind to Northrend, fly to Dalaran, take the portal to the Caverns of Time.
After all of this break your computer because you will find that you have wasted your life :)