Why are his feet perfectly flat on the bottom and blob-shaped otherwise? What magical light source can illuminate 90% of his head but leave the other 10% in dark shadow? Why are you painting on a bright white background? Why is his mohawk fully-lit even when it is traversing the same areas of the head that are in dark shadow? Why are his arms so short? Why is his adam's apple too low? What are those weird lines on his shoes? Why did you use reasonable hue-shifting for his skin tones, and saturated straight ramps for every other surface?
This piece looks very rushed and haphazard to me. This is very natural when you're a beginner, and not something to fear or embrace, but merely to notice and understand. The easiest way to self-critique and improve is to slow down just a little bit, and to look at every element of your image and ask yourself "why did I put this here?" I have tried to step in and be this voice for you here. You are not at a level where simple observations are going to result in a permanent quality increase in your artwork; they will be bandaids on a larger problem. You need to come at this with an analytical mindset; art is very subjective and creative, but improving it and correcting mistakes (regardless of style) is a matter of practice and discipline and self-honesty. Take a step back and look at every part of this image and consider why you made it look like you did. Was it because someone told you to but you don't understand why? Figure out why! Did you just do it because you had no idea what else to do there? Figure out what you should have done! Not sure how to shade this part? Look up references! Your work suffers from nonsensical choices regardless of style or aim, and some simple analytical thinking would go a long way toward correcting these problems.
You seem to have time and inspiration enough, you just need to develop the ability to examine your own work critically. You will always need input from others to advance your work (we all do!) but you should always be your own worst critic.
Keep at it!
