To remove a dent, first gain access to the back side of the body panel by removing items like headlight buckets, inner fenders, door panels and interior trim. Then, place a dolly on the outside of the dent and then hammer on the inside of the dent, striking the metal against the dolly. The key is to go slowly and work around the circumference of the dent, banging it out until it lies just below the undamaged area. After the dent is out, put the dolly on the inside of the dent, then tap the area surrounding the outside of the dent to knock in any high spots.
Really, if you don't know what you are doing with hammers and dollys you are probably just better off drilling out the panel and using a slap hammer on it. The holes help the Bondo grip the panel anyways. That, and fenders are like $50 at the junkyard.
You might find one of these useful in your future bodywork endeavours:
http://images.google.com/images?hl=en&q=shrinking+hammer&um=1&sa=N&tab=wi