Instructions
With a Flying Mount
Take the boat from Stormwind Harbor to the Borean Tundra if you're Alliance. Ride the zeppelin from Orgrimmar to the Borean Tundra if you're Horde.
Fly west until you reach Dragonblight. Go northeast until you reach the Crystal Vice. It's at the very tip of Dragonblight, so it will take you several minutes to reach. You've gone too far east if you see Light's Trust.
Travel through the Crystal Vice and under a small arch to enter Crystalsong Forest. Fly northwest. Look high into the sky and you will see the outlines of a city. Land in the city to arrive in Dalaran.
Without a Flying Mount
Earn experience until you're level 74. You cannot enter Dalaran until level 74 without a flying mount unless you get a mage to port you there or a warlock to summon you there.
Talk to the Kirin Tor representative at Valgarde, Star's Rest, Wintergarde, Valiance Keep or Amberpine Lodge if you're with the Alliance. The representative appears at Agmar's Hammer, Warsong Hold, Vengeance Landing, Conquest Hold and Venomspite if you're with the Horde. Accept the quest "The Magical Kingdom of Dalaran."
Speak with the Kirin Tor character again to teleport to Dalaran. Discover the flight path by going to Krasus Landing at the east side of the city when you arrive.