alliance, you go around the stormwind lake and there's a quest that sends you to moonglade.. you speak to a dragon there and they fly you up to it. I think there's a portal that takes you there after that.
Horde, you get a quest that sends to you moonglade again., it's near where the zeppelin towers are.
You stand in Stormwind or Orgrimmar, look at the Hero's Board, which wants you to go talk to a druid (for alliances, she is in the stormwind keep). She tells you to take a portal to Moonglade, and turn in the quest there. Then you take the quest to ride the back of a dragon into Hyjal. This dragon flies you by the Flame Spire (not on the beta, don't remember the name ) and some extreme things that I don't want to spoil will happen. The dragon then flies you to the quest hub, and you start questing.