get a long flatheaded screwdriver and grind off the edges so its nice n rounded, like this..
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take off any trim, door pad, whatever, so you can get to the dent, feel it and find out exactly where it is, and gently apply pressure to the dent and push around in circle motions, keep checking on the outside of the dent to make sure you aint pushing too hard or not enough.
be patient, do it slowly and eventually the dent will come out. first attempt you should make it look alot better, after that it comes with experience.
as for scratches, depends how deep they are, if you can only just feel them with your nail they can more than likely be flatted n mopped out (wouldnt recommend doin it yourself if you dunno what your doing), take it to a bodyshop n ask if the scratch can be mopped out.
as for surface scratches or key marks use G3, rub it pretty hard then go over it with something less coarse like G10 or whatever polish you use. then wax it if you like. G3 will get out virtually all scratches/scuffs/paint/tar etc etc
