...Oh, boy. You apparently have much to learn, my young Padawan.

Ok, stealing weapons is a Thief A-Ability. It is on the "Sword Breaker", which must be acquired through random quest/battle rewards or trading (you can steal it too, but that's kinda pointless...).

The clan skills raise by completeing missions. There are eight different skills, and each one has a different chance of increasing depending on the particular mission. As the skill levels reach certain numbers (ie: 5, 10, 15, 20, etc...), you will receive items.

I suggest you find an FAQ if you wish for more detailed info, because there are several different ways to get most of the items in the game, and I'm too lazy to want to make a post that long right now. Maybe tomarrow...