Specifics for Levels 21 - 30
* Story quests will help you level up VERY quickly. There were at least 2 quests that gave me about half a million exp.
* Do Vanguard Requests, these will have a few high reward dungeon quests.
* When you hit 27, which should be quickly if you do the story, you will unlock a new dungeon. Run that as much as you can for the newbie bonus.
Some helpful tips for leveling:
* Quests, especially main quests, the most obvious way. You should do main quests until level 20, then dungeons.
* Dungeons. Next to main quests. Lots of enemies and bosses, you can get a large amount of exp out of these (first dungeon is available at level 20, combine with the Rested Bonus (below) for even more exp potential).
* Vanguard Requests. These offer a nice exp boost and other items upon completion, and are available early in the game. Check often, as you get more rewarding requests with higher Item Levels
* Rest Bonus. If you go offline in a town, you start to accumulate rested bonus. Killing monster while "Rested" gives DOUBLE exp, quests do not grant double exp.