might just be me but it looks like you are pressing down suuuuper hard with your pencil when you draw. Try loosening up a bit, maybe draw something by doing lots of really soft strokes. That's what Andy Ristaino from adventure time does, and it gives his work a really unique style. http://skronked.tumblr.com/
Also you should try having some fun drawing basic 3d shapes so you get a feel for structure. Back in the 90s, there was Commander Mark's Draw Squad, where he'd teach all kinds of fun little exercises to do just that. http://www.drawsquad.com/lessons/lesslist.html try out these lessons, I think they'll help a lot.