Welcome to the Guild Marcario! GIMP is a great place to begin. It's fairly powerful, free and pretty well supported in the tutorials section here. As for tips and tricks, it's hard to offer these in the abstract. If you start a WIP thread and ask for feedback it would be easier to give advice. However there are some tips that can be given in advance. First, makes sure you understand the basics of plate tectonics, volcanic activity, erosion and climate zones. I think the adage "measure twice, cut once" is a good rule of thumb; you don't want to invest a lot of time into a map only later to discover that it doesn't make any sense. While there are discussions on all of these topics floating around (you'll have to do a search for them), here's one that is a sticky that you may find useful How to get your rivers in the right place. If you have any specific questions, don't hesitate to ask. Best of wishes on your new project.