Hiya Helios and welcome to Void.

You are playing around with a lot of the basics and thats great news, because as it seems you as of yet haven't found your true style. That can take a very long time in some cases or you could find it next week. Regardless how long that time is it's always good to know the tools and how they function.

On the artistic side I will point out a few key points that could help you decide what you do really like and how to better get it.

Try to work on position and lighting, these are imo the most important part about making a piece. Every thing in life casts a shadow, so without having shadow/lighting it makes it look off to the eye and hard to digest.
Currently you are working with renders, these already come with their own lighting from the original artist, so try and use what they have already given you to get something more real. A good example is your current tag, your renders lighting is coming from the far left and casting down to the far right, though he also has red light/fire coming from his explosion of his gun, so be sure to also use that lighting. As you can see he has dark shadow under his are and the right side of his face, thus telling the eye that it should see darker there, though your background does not show us this, thus giving off an unfinished look.

Second most important imo is colours, because signatures have such smaller space you really have to focus on your colours and allow them to match up so to speak to allow for the colours to draw your attention, again to the focal of the piece. Try to stick with colours similar to what you are already working with, again your render will help you out. imo you should stick to only 2-3 colours in one small piece allowing for fewer distraction Again using the example of your current tag, your focal has some really great colour to work with, and you already have started your bg with them, playing with the black/white scale is great, he also has red, and going into the blue spectrum wouldnt kill this piece either if you decided to go 3 colours.

Hope I have given you a little help in more ideas >.<