Seconding FireAlpaca as a good free program, as well as Medibang, which I think is made by the same company. They're basically the same program but FireAlpaca has a simple animation studio, and Medibang has more features aimed towards illustration/manga. Both free and both very competent programs. Medibang also has a real neat custom brush system, so you can make new custom brushes stupidly easy just whenever you need them.
I've been using Clip Studio Paint for about a year now and I would also recommend that for digital artists looking to take the next step up, it's pretty reasonably priced. About a million brushes available, and just a very natural feel when you draw. Probably the best watercolor brushes I've ever used digitally. It also has a linux version, I do believe? So if you're into that, that's a boon.
Oh and they recently added a timelapse function that lets you record you drawing and then export it as an mp4, which is slick as hell.