*Get Guildleves unlocked ASAP, they start accumulating as soon as you unlock them!

*Rest at an Inn or Sanctuary. Any sanctuary starts earning you bonus XP while you're in there. Staying at an Inn does it while you're offline. Go idle/afk in a camp. But if you're logging out, do so in an Inn.

*IMO, save your Guildhest bonus XP for a secondary class

Beat me to it! I was gonna write a post on this. Hahahaha

Yeah, use your primary class to go through Main Scenario and don't spend your Leve credits on it. Since the Phase 4 cap is level 20, you'll most likely reach it within the first day. Pick up your job's required secondary class and speed through it doing Guildhests and Levequests. Guildhests give you a huge exp bonus the first time you do them (and they are pretty fun and short!).

Here are the Job requisites (updated with information from the .dats)

Bard: Archer (Lv. 30), Conjurer, Lancer
Paladin: Gladiator (Lv. 30), Conjurer, Marauder
Dragoon: Lancer (Lv. 30), Marauder, Pugilist
Warrior: Marauder (Lv. 30), Gladiator, Pugilist
Monk: Pugilist (Lv. 30), Marauder, Lancer
Summoner: Arcanist (Lv. 30), Thaumaturge, Conjurer
Scholar: Arcanist (Lv. 30), Conjurer, Thaumaturge
White Mage: Conjurer (Lv. 30), Thaumaturge, Arcanist
Black Mage: Thaumaturge (Lv. 30), Arcanist, Archer

Remember you need your secondary classes on level 15 to unlock the job. However, it's well-recommended that you level them further if they have past level 15 cross-class skills that can be used by your job. For a list on that, check this

Good luck!