Are you asking for help getting started with skins?
Or, are you asking for help using GIMP to get the end result you want?
If the former, see the link in my sig. It uses photoshop for examples, but it will give you the basic steps on how to start skinning.
If the latter, that's going to be up to you to learn the program. Get used to the tools, and if you want to figure out how to do something specific, ask us (we may be able to help).