Airburst

If you're going to launch into fixing bikes, I suggest Sheldon Brown-Bicycle Technical Information and Park Tool Co. » ParkTool Blog next time something goes wrong. Sheldon Brown was where I learnt most of the basics. You might actually want to take a look at the Park Tools article on how to adjust rear derailleurs to make sure you don't feed your new one into the spokes like you did the old one. It basically reiterates what FB said earlier, but it has pictures, which might make it easier.