So uh..how many hours have you been spriting?
If it's your first, it's good. The two biggest problems here and the most common- shading and proportions. If you're just starting, I recommend editing from existing RIPPED sprites or any sprites you have permission to use. From there, you'll start noticing the size and shapes of body parts. With that comes shading. They go hand-in-hand.
Read this next sentence well and remember it: Spriting is trial-and-error and it takes time to get good. If you haven't, the best sites for sprites to look at are The Spriters Resource and Mario Fan Games Galaxy. Some notable spriters are CyberwolfJV, SMBZ1, joshr93, etc. You will find a lot of good work here on these forums.
All-in-all, keep working on this sprite and work on more.