LEVEL: 1-4 - Do your begginers quests by talking to the arua fairy.
4-10 - Go outside the little village and walk until you see choropies. (1st auto-quest) When you are done. Go back and find a little river. Across it you can find some Flanaes to do your next quest. And at the same spot you can find some pumpkins to do your last quest too.
10-20 - Go to Woopies outside of the city of Zant. You will find a lot of party and higher cleric level to buff you.
20-27 - Go to the Beatles in the Elverloon Desert. It's near the source of water. You will need a party here too to help you level.
27-36 - Go to the Aqua guards in the center of the source of water. There is usually higher level clerics and a party waiting for you !
36-48 - Go to the moldies outside of Junon city. You will be able to level by yourself or with a party. There is some boss you can kill with party too.
46-58 - Go in the the forest of wisdom and walk on the road a long way until you see Clowns. There is usually a lot of people training that you can meet and party with them.
55-65 - Go at the Kenji beach and fight some doongas out there. There is a lot of people training there, but you can level by yourself too.
65-85 - At Kenji beach, near the beach (^^), you can find Krawfies. Make a party with people there or join one and you will spend a lot of time hunting these monster.
80-90 - Go to Goblin cave (Located in the adventurer plain). Get to the 2nd Basement (B2). Kill the Goblins worker that are just near yourself.
90-120 - Get to the 3rd Basement (B3). Find a party there and train here until you are level 100. There is no specific spot. Just find one where a party is and join it. When you are level 100 do your 2nd Job quest and come back there after.
110-125 - (Optitional) You can go to Luna, teleport at the teleport chamber near the temple, and walk up a long time until you find Hell Vulcans. They are grey, but give 250k xp per kill so it can be good. (I sincerely prefer training at the Goblin cave even if goblins are grey too)
120+ - Experience by yourself or go to the Oblivion Temple. My characters are lvl 123 so i can not tell you where to go now xD !
150-180 - Sikuku Underground Prison.
Have fun and sorry about my english, it's my 2nd language