Only advice I can give at 14 is keep drawing. Draw everything, look hard at stuff, read books, get inspiration. Learn how to see. Using Photoshop is a useful skill, but learning to see and draw will help you in every aspect of design.
Think about shadows and highlights on the guys face. At the minute it looks a bit flat in places and stands out (mainly his face). The coloured area should be a little darker overall because he's lit from the back. I'd also soften the edges the layer too so that it's not as sharp and jagged.
Keep at it though. Getting form (the 3D shape of an object) right takes patience and experience.