The usual advice is to get a panel from a scrap-yard and practice on that before going near real cars.
My own piece of advice is this. It will seems obvious now but, trust me, at the time it will not. If you do mess up and you have somehow added scratches rather than removed them, don't try and fix it. A decent pro can almost always recover one misadventure with a machine polisher. Trying to fix it yourself can result in repainting being required. Think of Father Ted's dent. Good luck.