MyPaint is an Open Source painting and sketching application that has versions for Linux, Windows and Mac. It is made for the graphic tablet and is quite intuitive meaning, without fuss you can start making scribbles. Learning how to use it to do something with it however takes time.
I want to be a painter, I buy brushes, tubes of paint, canvas, great desire and go to it. I squirt, I brush, I dab, I smear and discover I needed to prepare the canvas first, then proceed to paint with forethought mingled with invent as you go. I'm not knocking traditional media, just that the process is the same but different for PC art. Put things in good order so as to not waste valuable art materials. On the PC it is not so much a waste of materials but time if that is important to you.
I was sitting in the gallery office, had the tablet hooked up and trying out the brushes and backgrounds in MyPaint. Each brush lends itself to a kind of line, texture and evokes a look. Just like with real paints you have to discover what you can do. There are movements that are natural to you, those are sweet spots. There are awkward movements, those are challenges.
Like I said many times I want to start by drawing like I do in my sketchbooks, why? That is what I am used to, sweet spots. Then try to move on. I started with a few pages of brush stroke lines, smears, splotches. Then tried to form some shapes of objects. A picture started to take shape. Ooh, the eraser works and the undo and also the save often (learned that from drafting).
Subject matter is your passion, superheros, spaceships, animals, faces, you know what you want to do. If you give it a start no matter how cock-eyed it looks, eventually you'll work towards a good outcome. Me, I'm not a natural artist which is rare. I'm a home-brewed, dyed in the wool doodler. If you have professional artist desire, go for it, I'm in it because it's fun and I like it. I have a romantic notion of the fine artist of the past. Today most think of a JOB, get some dime for your time. Keep your passions in sight, prepare for opportunity but, do it because you like it.