New Camera-Still Poor Pics

Actually, a little taped ring of one inch by half inch postcard might work well (you might need a larger piece to handle larger coins)... but for the background to go out of focus *completely*, you have to have a *lot* of light - allowing for the widest aperture your lens can manage. =) Since you need a lot, clean white light is of course best.

Generally, the default position for most cameras auto exposure setting is "center-weighted" - this should work well, so long as the coin takes up a sufficient area of your viewfinder. If not - or if you want to get picky - see if you can set your exposure manually so that you get consistent results. You want to set it such that your aperture is at it's widest (or numerically, smallest) setting while still providing an image that has pleasing and useful contrast.