To give you a little context for the options that have been presented, Gimp is what we call a "raster editor," which means that it works with grids of pixels. It is good for images that look photographic or have complex textural detail. The satellite style on Google maps is a good example of raster imagery. An image created in a raster editor is locked to the size it was originally created; you can make it a little larger, but not much before it starts to lose quality.

Inkscape and other vector editors create images based on mathematical formulas. They typically render smooth, crisp lines, colors and patterns. The map style on Google Maps is a good example of vector graphics. A vector image can be scaled freely without losing quality, but great complexity and texture is more difficult.

Sketchup is used to create relatively simple 3d graphics (although there are some power users who can make stunning photoreal images with it). If you have the Google Earth plug-in, you can see such 3d objects in major cities on the map.

Whichever route you choose to go, feel free to post a Work in Progress (WIP) thread here and ask questions. You should find plenty of people willing to offer advice and assistance as you develop your maps.

Welcome aboard!