This is pretty bad, but I'm working on it. Any tips on better ways to use GIMP are much needed. thanks
First, crop the extraneous background so you can post the actual coin itself larger. There's no need to include excess background. Next, perhaps a second light and place them at 10:00 and 2:00 (to start) more vertically above the coin rather than a single light down and left of the viewed face. Half of the coin is in shadow now, and the the Gimp cannot help that.
I prefer Photoshop to GIMP. You can download PS CS2 for for free from Adobe. I like the cropping tools better in PS than in GIMP. However, what Dave says is true - you need to take a solid picture before any editor will help.
I've found Photoshop and the GIMP to have long learning curves and since I'm tele-pathetic and a peripheral visionary I don't like learning curves. Paint.net is free and has a very intuitive UI. I felt comfortable with it from day one.
Here's some basic tips - without me knowing what level of knowledge you already have of course! Duplicate the layer (ctrl or cmd + j), use auto levels and color balance, duplicate again a few times. Now play around with the layer composites, such as multiply, screen etc, working from the top layer. Depending on the quality of your source image, you can really bring out the detail of your coin. And try and avoid circular marquee selections if you want to mask out the background color, as this gives an artificial looking roundness to the coin particularly if the edge is fluted. Hopefully not too off topic but it is all coin related!