I started by drawing in MyPaint because it was very user-friendly, simple, and easy to learn. But its functionality wasn't great, unfortunately.
I already knew about Krita back then, and I gradually switched to it. This program has a lot of hype because of David Revoy, the guy who draws the "Pepper and Carrot" comic. Its charming style immediately makes you want to learn how to draw the same way.
Actually, I also used a guide by him. David emphasizes using as many layers as possible with different shadows, recommends brushes, and shows how to combine shades of gray into color. It's quite helpful.