basically, you start off with warrior and mage. as you level up your character and get the required (insert job here) levels you earn new ones your able to use. all jobs start at level one. no exceptions so if you deside to leveling another job you got a bit of a grind ahead of you. Also, there are 2 kinds of skills, ones you get when you level up - they are NOT the same for any one job however you can level them right at the start to help level the class, the limits for these kinds of skills is the same as your BASE level. the next kind of skill is the ones you get when you level your job, same thing with the other type, cap is your current JOB level for that job. now onto certificates or whatever they're called, you seem to get them every 10 JOB levels for any job. they're passives that you can use with any and all jobs, but you can only have a limited number of them on at the same time.
sorry for wall o' text