There are some complexities to the job system of Final Fantasy III. There's a lot of things you need to know before you actually change jobs and the reasoning behind them. You can also serious inhibit your character's preformance / maximum capability if you stay in a job for too long and grow too many levels before you change out. Basically you get the ability to use the job system and switch out once you've removed the Dijin's curse and re-met the crystal's voice again with your full four-person party. At that point you should decide which characters you'd like to be magic or offensive based. It would be better to make all of your characters balanced, but you can choose whatever you'd like - to be honest. However, let's get talking about the basics.

To change the jobs of any character, go the menu and click on the 'jobs' tab. You'll then have the choice of changing your character's job. The job levels are listed right by the job. Job Levels are exactly like character levels, they determine the statistics for your character when they are on that specific job. Increasing in job levels changes your proficiency in that particular job. If you are a warrior, and you grow a job level, your attack power and your number of strikes per attack goes up - next the defense will also increase.

When you've been on a job - especially for a long time, adjusting to a job can be difficult, especially if it's a change from magical to offensive or vis-versa. Doing this can especially inhibit your chances at surviving in a new area. As well, you should spend some time battling to level up your j.lvls to adequtely battle through everything.

If you'd like to max out your stats, especially HP, don't stay on a magical job too long. This can affect your ability to max out HP, because of the magical jobs' inherent less HP growth.