Ignore the stars. I don't think they're really relevant to critiquing... DA just put them there because they look shiny or something.
I'm critiquing this in order to give you suggestions on how to fix the issues, not just tell you what the issues are.
The other critique was correct in that there are some obvious anatomy issues in this, but that's normal for your age. My anatomy wasn't much better at 14. I can see you're going for a style here and while it's more solid than most styles I see from kids your age, it still needs work.
The best recommendation I can give for developing a style is to learn the textbook proper way to draw anatomy first. Once you learn those rules, then you'll be able to bend them into a convincing style. All styles are rooted in realism in that way, because that style's reality is actual reality bent into a different shape, if that makes sense.
You have the right idea going for how to foreshorten the arm, it would just be more convincing if you had more of the shape of the forearm in there leading up into the upper arm. The hand also looks pretty good for you experience level... the index finger is just a smidge too long.
On his right leg, the thigh is a little too thin going up into the pelvis. That's where the thigh should be the thickest is as it's connecting up into the glutes, so if you bring that line down it will help a lot. The left leg looks broken right now because the knee doesn't bend like that when the leg is in that position. I see what you were going for, though. It will look better if you either swing the knee out toward the camera to further foreshorten the thigh, or else turn the foot inward so the toe is pointing more toward the corner of the image. The leg DOES bend in that direction so it will help with it looking wonky.
As for the background I would suggest lightening it up some. What happens when you have colors that are THAT saturated behind a light colored figure is that the background visually jumps in front of the figure and overpowers it. The first thing my eye is drawn to in this image are the sunglasses on the background figure rather than the figure in the foreground. Make the background image (including the sunglasses) lighter and/or more transparent and that will help to push it into the background where it belongs.
For overall study of anatomy that is relevant to artists I recommend Atlas of Human Anatomy for the Artist by Stephen Rogers Peck. (Amazon.com: [link]) It will be a little advanced for your age and current experience level, but you'll definitely grow into it. It lays out the superficial muscle and bone structure that is relevant to artists and breaks it down into sections so it doesn't overwhelm you. It also has text explanation of the function of the muscles in that section of anatomy and some incredibly useful sketch pages by Peck that shows proportion, movement, and general shape reminders that will help you remember and better understand how to draw that portion of anatomy. That book was the required textbook for a Figure Anatomy class I took my senior year of college and even five years after graduating it still stays right next to my desk stuffed full of post-it notes, dog-eared pages, and highlighter bleeding. It's pretty cheap, too, which is a bonus.
Overall you have a nice idea going here and a pretty decent style developing for your age. You just need to focus on learning proper anatomy for a little while and I promise it will make your style stronger in the end.