Sure, all you need is Gimp. I've never used it but it works for many. Any photo editing sort of program will do. I use Clip Studio Paint. It's often cheap on Black Friday.

Popular software specifically FOR mapping includes things like Wonderdraft (I haven't used it) and Other World Mapper (I have used it and have a few recent maps posted using it, since you can set a grid up in it it would work excellently for Roll20 maps).

The only things I've bought are resources, like texture packs, and programs. I've never paid for knowledge. I have paid for Other World Mapper, World Creator (a 3D map program), Affinity Photo and Designer, and I plan on buying CrazyBump and Lazy Nuzmei. If you use Windows the last one is a stabilizer and ruler kit for digital art, very useful, and pretty low cost.

Hardware, a tablet for sure. I've always gone Wacom and I've worked up from a 6x8 Graphire to a 24 inch Cintiq. Every model I've tried has been reliable and hasn't broken before I upgraded. I've regifted several tablets due to their sturdy reliability. A used tablet is often fine, you just might want a new pen.

Look at lots of maps. Like, lots of maps. And draw lots of maps. Yeah, that's basically about it.

Also, Old Maps Online.