While I aggree with the need to tinker with the progs to get comfortable, I don't think you need to be comfortable to start mapping. I think the best way to get comfortable to to try following the tuts and see what you get and then tinker. This way you learn the prog as you go and what the various changed actually do.

For Gimp, I cannot recommend RobA's tut on creating regional maps enough, and you can find it here. There are many others to walk through but this one has soooo much stuff that it teaches you about Gimp. The style may or may not be what you are looking for, but that is where the tweeking comes in.

Have fun, and post your work as soon as you have something to show. It need not be complete either. The WIP threads are a great place to get over any bumps you find along the road to great mapping.