Oh god where to start. First off, let me say the dark souls wiki is a great source for all your dark souls answers. to get a basic catalyst you can talk to the undead merchant in the undead burg and buy a house key from him. after that you beat the gargoyles and go to the lower undead burg. after going down the staircase and killing the dogs go to the right and you'll hear someone complaining about their situation. Open the closed door on your left and you'll have freed the mage. The next time you return to firelink the mage will be there and he will sell you various starter spells, a couple rings, and a catalyst. It is one of the best for low levels, surpassed at around 20 Int. by the Oalacile (spelling?) catalyst, and at 40 Int. by Logan's catalyst. Again, the wiki has all your answers and more.