If you haven’t started the Legion quest line, you should be able to pick up “Legion Returns” from the mission board in your capitol city. If you’ve already completed the intro on an alternate character you should be able to skip it and be ported directly to Dalaran (with the quest “In the Blink of an Eye”, which you’ll turn in to the NPC there).
Some of the comments are a little out of date, but this may help: