I just learned the crafting system here - didn't really do any of that in my brief jaunt in wow. Here's the gist. Your companions get 3 crew skills (you can see this menu by pressing N). There's a main crew skill. Based on the main skill you pick, you can create armor, or weapons, etc. Then, the other skills you get are typically used in support of that main skill. Take for example my level 24 sith. I choose my main crew skill of artifice. This enables me to craft lightsaber parts along with shield generators. I picked this crew skill because I use lightsabers and I use shields, so its quite useful. If I didn't use lightsabers, though, I don't think it would be exceptionally useful. Anyway, I choose treasure hunting and ... - can't think of the name right now). Both of those support skills create missions for my companions. So, I send them on missions and they get me the parts I need to use my artifice skill. So, they bring back the parts, I craft items. I can then reverse engineer the crafted items and before too long (by repeating), I'll get a new blueprint for an even better item to craft. Anyway, that treasure hunting skill is even more useful, though. With it, you can search for companion gifts and more importantly, lock boxes that include items or gold, etc.
Anyway, the more you use any of your skills (and if they are successful), the more experience you gain in that skill. So, if you do it a bunch, you eventually get access to better crafting recipes and better missions (for instance, on my level 24 sith guy, I can get very good level 3 lock boxes that often include some VERY good items).
Anyway, that's the basic overview. I'd suggest googling it to find out what might be the best combination for the character type you are using. I'll be doing that with my bounty hunter as soon as I get my companion.