Well, Xaria (Daenerys Targaryen) is really good with banners, so here's a tutorial that could help you:
http://www.fantageforum.com/t10681-the-basics-on-gimp-an-introduction-to-gimp
And look up some tutorials on Google, also. I don't know how to create banners myself, so I apologize, since I don't really have any advice!