The best free option I know of is Krita, it's pretty decent and definitely good enough if you're just getting started.
If you want something a little more professional, Affinity Photo is by far my favorite app for painting and drawing. It's a one-time fee of $50, but you can get a 30 day trial if you just want to check it out, and it's definitely worth the pricetag.
As a side note, none of these apps are going to do you much good if you don't have a graphics tablet. The one I use is a Huion 1060+, which I got for about $65 about a year ago. It's a bit buggy but more than decent for the price. Smaller tablets can be even cheaper, I've seen them as low as $30 before.
Hope this is helpful!