The above post is a great start. I find it easiest to start "big" and then work down. I also find that it is helpful to have a fairly solid idea of what you are trying to create. Answer some questions about what you are hoping to accomplish with the map. You don't have to know everything at the start (I NEVER do ) but it is helpful to at least have a guide as to the large features of the map such as mountains and large bodies of water. Map making is as much about the journey as the destination and every journey starts with the first step. Draw something roughly (either by hand or using GIMP) and ask your self if there are any features that you absolutely have to have on the map. Draw those in. Then ask yourself what might be happening around those features when you have the answer, draw those features in. I cannot emphasize enough how helpful it is to hand draw your first rough outlines it will give you a guide to work from and give you a place to jot notes and other things on. Most importantly post your work in a WIP (work in progress) thread in the appropriate forum section. There are many GIMP users here (myself included) that would love to help.