Basically you need to make the surface you are painting able to take paint... but by 'keying' the item you are also cleaning up the surfaces.
get hold of some water paper 120 grit and 180 grit rub down your item for paint first with the 120 (rougher) you can use water on the paper to make life a bit easier but you need to be sure that by the time you have finished it off with the 180 paper you are ready to clean out all the sanding swarf you will create... then you can go ahead an spray it. it needs to be noted that if the surface aint too bad you can generally just give it a clean with white spirit, let it dry fully and then just get a couple of dust coats on it and build up your paint slowly, that should give you a pretty good finish... alternatively if you know of a good powder coater that is an easier but slightly more expensive route to take... the finish with powder coating will be beautiful and probably more hard wearing depending on the quality of the coaters work...
good luck mate.