Hi Windspear, as this is your first post and a question about tips to mapmaking, I've moved it here.
First, a very warm welcome to the guild! I hope you find it helpful in fulfilling your mapping ambitions.
GIMP is great for making maps. If you look at the tutorials section of the site, you should find a number of tutorials about mapmaking in GIMP many of the photoshop tutorials will work with GIMP too, with little or no tweaking. I suggest that you try RobA's tutorial on making an artistic map with Gimp. You can follow the steps and have a map at the end of it. Also try Don's Enenei Medieval city tutorial. Don's tutorial uses photoshop, but you should be able to follow most or all of the steps with GIMP.
In terms of software, there is lots to choose from (including not using software at all, but using a pen and paper), it depends on what you are most comfortable with. Take a look at the link at the bottom of my sig about the software you are using, that has discussions about what's available and the pros and cons. There is also a sticky at the top of the software discussion thread which talks about different software that people use, the price of it etc.
Enjoy your stay at the guild!