You need to go youtube and start watching tutorial videos on how people using photoshop or other related applications. Then go to whatever you are using (gimp, Photoshop) and start mimicking what they are doing. Once you get basic technique down, before you know it you will develop your own style. You will learn from observing and experimenting. No one can sit down and teach you to be better, this is a fallacy and if they can it will take an incredible amount of time which I'm guessing you don't have.
You wont be able to paint like that immediately of course, these videos just give you an idea of how painters start their pieces and what techniques they use.
For example: notice how they use low opaque brushes for the rough stuff, the beginning of the painting, the underlying structure (I hope this makes sense) and the opacity slowly increases as they go into more detail and are closer to finishing. So you can go crazy with low opaque colors because if you mess up you can just paint over it. Again you need to start practicing, there are no shortcuts.
You can also go to CGSociety.org and look through peoples portfolios, look at what colors they use, shapes, etc.
Here is one such portfolio, the more you look at, the more stuff you will have to experiment with when you are working.
http://bluefley.cgsociety.org/gallery/ (at the bottom you can find their paintings, and then other portfolios, make sure to pay attention to what they are using, this site deals with 2d and 3d art)
Also if you have a Facebook account, get the graffiti app and replay other peoples work. I mean go into the "best of" gallery or whatever and look at the replays. Those are also a good source of how people work and use color and opacity.
I'll do that when I'm home, cause my. Internet is real slow here.