I'm pretty sure firealpaca is free, but I've also never used it, hope this helps :D
I've heard good things about Krita but I've never used it myself. Medibang is good for starting out (kinda barebones in my opinion though) gimp is great, and has a lot of photoshop-ish features. I'm pretty sure firealpaca is free, but I've also never used it, hope this helps :D