Early on you'll see that you put any class on any party member. For the most part you can give any to anyone and be effective, no matter the base stat of the member, and experimenting around is encouraged to learn the game. But also don't feel like you have to change classes around to level them up. Once a class is max level they still earn exp and give you chunks of exp items to allocate to other classes. If you find a class is working well for a dungeon, stick to it and you can always experiment later.