probably alchemy. you can pick up some items that restore health (meats, some plants) and brew them up in your mortar & pestle. actually, alchemy is pretty good for making money early on. you can just wander the countryside harvesting random ingredients, mix them into a potion, and sell them for 100% profit

there's also some healing spells in the restoration school of magic, but imo they are very ineffective for how much magicka they cost

>how do you join the mages guild?

i'm pretty sure you've just got to find one and talk to the right person. ask about 'mages guild' once you're inside and they'll usually tell you who the leader is