Traditional xp parties haven't really been a thing for a while now. So important to start collecting Trust NPCs as soon as you can. There are repeatable objectives that you can undergo that will give you a small xp bonus every time you kill X number of a specific type of mob, called Fields/Grounds of Valor (one is for dungeons, other for open areas). Once you start collecting Rhapsodies key items, you'll start getting increased xp so that won't be as important. Since parties aren't really a thing, you're free to go where ever you want to xp, but will probably want to focus on things you can do GoV/FoV pages.
Edit: Many Trust NPCs have limited availability, making it even more important to start collecting as soon as you can. They'll be switching around the available trusts very soon.
There are two major wikis that I use to find stuff. The ffxiclopedia is good for older stuff and more general information. It hasn't really kept up with new endgame stuff that well. BG wiki is more focuses on endgame.
http://ffxiclopedia.wikia.com/wiki/Main_Page
https://www.bg-wiki.com/bg/Main_Page I can't really recommend any guides to levelling at this point, I haven't done such a thing in a very long time. But if you look at the list of FoV/GoV you can get an idea of what's available in your level range and give it a try.
http://ffxiclopedia.wikia.com/wiki/Field_Manual
http://ffxiclopedia.wikia.com/wiki/Grounds_Tome
Edited, Dec 26th 2015 10:04am by Nanako