No difference than learning any other skill. Set specific goals and choose your exercises based on those goals.

I highly recommend learning basic drawing skills, which may be easier to practice with pencil and paper - perspective, lighting ("shadows" is a misnomer), construction (how to draw 3D form in 2D). Studying basic anatomy of the relevant species will also help no matter what style you go with.
The reason I recommend pencil and paper (or any other medium you're already familiar with) is because it's much easier to learn and practice one thing at a time. If you're trying to learn perspective while also struggling with pixel line quality or with making detail in a small space, you'll just end up taking longer and getting more frustrated.

Pff, books xP While "Loomis" is a common recommendation and I can't deny you that knowledge (here you go: http://www.alexhays.com/loomis/), I found other resources to be far more useful:
http://androidarts.com/art_tut.htm
http://www.itchy-animation.co.uk/light.htm

While "drawing as much as you can" isn't a bad idea, you should try to draw in ways that will allow you to exercise skills you need to learn. You should also try to get feedback on your art (and specifically, your success in the skills you're trying to learn). Doing 10 drawings with a clear aim and with self-critique (and preferably external critique) will teach you far more than 1000 aimless drawings.

Learn to see like an artist, and never stop. I don't just mean the whole "draw what you see, not what you think you see" thing, I mean observation in general. Start looking at the world instead of just seeing it. Think about why things look the way they do. For example, the colour of green leaves on a grey day is different from their colour on a bright day, or from the colour of those leaves if you put them in a bright red box, but your brain automatically tries to do "correct" the colours you see to make the leaves look the same. Try to look past that, and see how the colours mix, and where each of the contributing colours comes from, and how different objects react to similar lighting situations. People who don't do this are the ones who struggle with things like drawing night scenes or whose sunny daytime scenes look overcast no matter what they do xP

Look at how things look and try to understand their general structure (trees, animals, cars, buildings, etc), because it always helps to have an idea of these things when it comes time to draw them. Look for patterns. Do tree branches tend to grow in specific ways or start at certain heights (depending on the type of tree)? Do buildings tend to get built facing a certain way? How tall are various types of vehicles, compared to their riders? Knowing these things will help you draw them in a believable way later, without spending as much time looking at ref and without having to have other artists point out to you that your trees look like brooms and that your cars are oversized xP You may be able to get this understanding from looking at ref photos too, but if you make a lifelong habit of understanding the things you see when you're just out and about, you'll save yourself a lot of time later.

Some people carry around a sketchbook and try to sketch everything they see in it to help them understand the structure, but I personally find that distracting, because it means focusing on the specific objects you see, rather than the patterns between them. Sketching from observation is great, but I find it to be best as a separate exercise from this.

I wish to say that I admire that you keep at it and work harder all the time. You may not be the best artist (yet) but your attitude is awesome.
There is nothing more to do but continue practicing and experimenting for us all. Kutgw.

By the way, i thought i would find more programmers partners around here. I'm surprised.

Well, as long as it isn't web related, i have done some things. I have been programming for some years now. What is your medici project about?

Ah! i found the topic. I didnt know it was a game, where can i get it?

That's fantastic because that's the kind of game i want to make and i'm currently doing. But i barely have time for myself. If i have free time in the future i may contact you. Maybe i cannot do the game, but can give you some engine so you can edit map/add events, etc.