NPC Questions.Follow

How do you know what job its useing and how do you change it? Can you just give him any weapon you want, or do you have to do a quest to unlock the equiping stuff? How do you know his lvl?

Yep, that guide is the best.

But in short:

go to a Rendezvous Point (Jeuno's is in Ru'Lude a little west of the signet guy; Windy's is by the mog house in Waters)

When you're there, you can find out his level and fighting style (attacker, defender, healer). Later on you can get 3 "advanced" jobs for your NPC, which are improvements on those first 3 types. While these jobs get basic abilities like double attack, provoke, and cure (depending on type), the weapon does not determine the job of the NPC, and he'll never get DRK abilities like souleater or anything like that.

Depending on your bond with your NPC, you can trade a weapon to Luto to give to your NPC (I think you can also trade directly, but I've never done this). You don't always have stay with one weapon type. You can go from a Lvl 1 Dagger to a Lvl 2 greatsword to a Lvl 3 katana. If you do change the weapon type, don't go "down" in levels. If you are at level 4, the next weapon you trade should be lvl 5, regardless of type. The complete list of weapons you can trade is on that website. In general, the Great Katanas have the best damger-delay ratio.