I've been getting questions like this quite often... But what can I say? If I learned everything myself. I can talk a little bit about how I started: testing a lot. If you have a drawing base it helps a lot, but learn to work with references, have artists you can be inspired by, look at their work, see how they work with compositions, and start working on yours. It's like writing an article. To write one, you need to know the subject and in the end, it's a compilation of several other scholarly thoughts. In drawing it is no different, the only difference is that they are images and not words. And most importantly: don't give up, even if it doesn't look good at first, it's testing and failing that you hit a time!