Well you know what a job is right? It's a class, like Black Mage. You are allowed a main job and a support job. The support job acts as half the level of your main job. So if you are level 20 Black Mage and sub White Mage you will have access to every spell and ability a level 10 White Mage does (assuming you leveled White Mage to 10). To gain the ability to equip a support job you need to do a quest in the town of Selbina at level 18.

Ranks you get from doing storyline missions. You start at rank 1 and the highest for each nation is rank 10. They don't really mean much these days but they are fun and there are various benefits from doing it.

Right now you should be fighting crap in the areas outside the starter towns to build experience. At level 10 or so you can start to party, but it might be quicker these days to solo/duo your way to level 18 and go after your support job. As you get higher in level you can venture further from town for harder monsters and better experience.