Hi there Orcish Lover, welcome to the Guild! Well as a GIMP user myself I can tell you you that you have come to the right place. There are allot of tutorials for creating maps of varying styles in GIMP located in the Tutorials section. Best thing to do to hone your skills is try a few of them out, post up work-in-progress threads and ask for advice. This is a very helpful community and your bound to receive constructive feedback.