Good a new one, maybe I can save you from going down the wrong path here. Stick around and post all of your stuff. We'll worry about the specifics and the content later on. I've been doing this digital art gig for awhile, believe me, you'll improve much much quicker if you stay away from making signatures, and if you stay away from using downloaded brushes and using pictures like game renders, images from tv shows and movies etc. Stay away from all of that, so that you are making 100% everything on your canvas. Work big as well, do not do signatures. I've been doing graphics for a bit over 4 years and I've made probably 4-5 signatures total.
The reason you should work big and completely from scratch is because when you do that, you make it much harder on yourself. When you are challenged like that you come upon a lot of problems, but in the process, you learn to solve those problems. You'll race around the learning curve much quicker that way.

So, to sum up, work big (bigger than 600x600 every time) and make 100% of the stuff on the canvas. No matter how crappy it turns out at the start, do that. Stick around and we'll help you improve on the content and the specifics of your piece so you develop a better understanding of aesthetics, what colors are right, what font to use etc.