Confused about classes

[Naria]

You can level all classes on the same character. Once you have completed your lvl 10 CNJ quest you unlock the ability to change class. To level arcanist all you have to do is go to the arcanist guild in Limsa and do the "So you want to be . . ." quest from the guildmaster there. Equip your arcanist weapon and boom! you are a lvl 1 arcanist.

You can switch between classes at anytime you are not in battle. You should make gearsets for each class you unlock to make switching easier (Character -> and then hit the gearset icon to bring up your gearset list). After level 30, parties will start expecting you to be a White Mage not a CNJ so I'd recommend leveling arcanist to 15 sometime before getting CNJ to 32. As for leveling Thumaturge, you can hold off on that for now, but post lvl 50 you will be expected to have swiftcast crossed class on WHM as well (partially for in-combat raise).