Unfortunately for you... There's no clear cut way to balance anything. Not classes, not skills, not weapons, not anything. Balance is what you want it to be within your own game. Balance is usually determined by how quickly you want enemies slain in combat and how quickly you want your characters to attain power.
If you're using any of the defaults with the program... You're going to notice one thing quite quickly. That is, the defaults aren't really designed to make a game that lasts more than 5 hours max. Default character stats can plow through the first dozen or so monsters without issue, and the rate at level up means they'll be gaining many more very quickly. Likewise, the formulas on the skills render any kind of hit of "Attack" meaningless because of how much damage they'll do to most anything.
If you want to balance classes specifically, you need to decide what the roles of each class is meant to do. How useful it is to a player (not yourself, to an actual player... you'll have to get outside help in playtesting in order to properly determine whether or not your classes are going to be used by players)? How useful is it in combat? How about outside of combat? What would make the player want to take one class over another? Are all classes equally viable in an RPG situation so you aren't running into "the holy trinity"?
A lot of balance is determined by the creator. There isn't some guide to help you figure it out. Often, it's just asking, "is this too powerful?". It's a lot of trial and error to get the vision of the game you want to make.
