Once you start unlocking dungeons, those will be by far the fastest way to level up. (Especially if you stick with Marauder as your main class. As a tank, you can get a spot in dungeon groups much faster than damage dealers.) Don't neglect the main scenario quests, though, because those are what unlock new content, as well as having the main story of the game.

If you haven't reached the first trio of dungeons yet, then just focusing on the quests is the best way to both level up and advance to where you'll unlock them.

Besides dungeons and quests, your hunting log and FATEs are both good sources for XP. (And once you join a Grand Company, the FATEs will also become a major source of GC Seals.)