i've asked the same sort of question before, and got the same sort of result that you have. The thing is that it's really difficult to give those sort of tips, it honestly does come from trying and trying, and each new painting you do, you'll learn something new. Firstly what sort of painting style do you want? do you paint realistic or cartoon etc?
some things i learnt along the way:
The things i struggled with was the canvas size, if you are doing a 'proper' painting make the canvas size around 3000 pixels, have the resolution at about 100 to start, you can increase as you start detailing.
People differ with the opacity and flow, but i personally keep the flow to about 60% and the opacity to around 40% as i couldn't work with full flow and opacity (this is a vague guideline, you'll find your way)
to build shades just go over and over an area, colour picking to get a smooth blend.
Don't use dodge and burn too much (if at all) you can achieve the same results using just brushes.
don't shade with black, or highlight with white, it doesn't work.
i think that's all for now.
keep posting as you go along, and welcome to the forum