Two words. "practice" and "study".
The problem doesn't come with knowing what to do. As it is with knowing just how to go about doing it. Everyone and their mother can tell you to "practice more", but if you don't know how to practice. Then you're pooched from the start. So just how do you go about properly studying/practicing? Well. How people learn varies from person to person. Hence why common day school fail so often. Not everyone learns the same way. So take this with a grain of salt. A common way for damn near everyone to improve, is by simply drawing/painting what you see. For instance. If you're drawing someone sitting up against a tree. But can't get the composition, angle, proportion, etc. right. Then simply go online (or outside), and find an image of someone doing just that. Then simply draw what you see. This will engrave it in your head as to how to do it in the future. Remember, when you draw something from imagination. All you're brain is doing is compiling all the images its seen of that (or a similar) situation. and than displaying them for you to copy from memory. So you literally can't draw something, without referencing it in some form or another. Even if you don't think you referenced it, you did, and just didn't notice.
That said. Never (and I do mean never) listen to people who say "real artists don't use references. They can draw perfect from the start with nothing but their mind and and a sharp stick, blah blah blah". Now while I'm not sure why you would need a sharp stick, I can guarantee these people are talking out of their ass. As it makes them feel big making it seem like they never do the very thing they rag on others for doing. When in fact they do, and are full of complete shit. Everyone starts with the basics, and progresses from their. Anyone who says other wise, is either lying. or. Struggling with progression like a baby in a bear fight (Is it just me, or does that seem like something you'll see on fox in the near future?).
When it comes to improving painting skills (real or digital). Honestly, I struggle with this my self. Its more or less the same thing as improving your drawing skills. It just requires a lot more attention to detail (shadows, lighting, colour theory. etc) compared to basic drawing. As far as getting the proper techniques and things like that. The two best ways I can think of, are A: Just dive in, and do it by trial and error. or B: Watch videos via youtube or other sites that show you how to do it, and simply follow along.
The key thing to remember in all of this. Is don't get discouraged by something not coming out exactly how you wanted it to. Just take it as a lesson to help improve. Study where you went wrong, and where you went right. Then apply what you've learned to your next drawing. I fully believe the hardest thing any artist will ever have to learn, is how not to be so hard on them self's for something not coming out perfect. As Chuck Jones, creator of Wile E. Coyote and the Roadrunner said, "every artist has at least 1000 bad drawings inside them". So for every one that goes tits up. Just take that as one step closer to that gleaming light of perfection.