Theres no real depth or shade to the marines, youve rendered clouds over a traced outline of the halo marine, then placed black vector lines over the top of them. Its always better to be orginal with your work, starting with your own lineart or traditional work and editing it in a graphics package.
Look at painting tutorials and decide which style you think suits you. Theres, vector, pixel, matte and quite a few others to choose from. Even starting with cell shaded 'chibis is a very good route for character development.
For a basic start,
Try free handing smaller less detailed characters so you can get the hang of movement and dynamic poses/standings. After that, work on emotions through ethier facial epressions, movement, or scene settings. At this point look into colour and how it effects the mood of a piece. Then its lighting to give the peice a real feeling. Then to anatomy to get the character physically right, If thats the path you want to go on. Theres hundreds of routes to learn how to paint properly in digital art.
If your after character design then a two fav's from my mini libary would be,
100 ways to create fantasy figures -
Link
Force, dynamic life drawings for animators -
Link
Both are at the top of my reference list next to bridgemans works and bold visions. So i would reccomend them for getting a good understanding of pretty much everything ive said up there, But the chibi. Just google that
I cant say i structured that well, but after soldering and tinfoiling my keyboard back together after crash rage on a essay, its understandable.
