Hey,that's a nice start.Kinda big for an early piece, but you have a template in this case so it's not so bad. Here's a couple of quick suggestions.

Here's what I think the skeleton should look like for this pose. The legs in the image are a big wonky. It's kind of like he's raising a foot, but it looks weird from the angle of the leg. His left arm also probably shouldn't be so close to his head, or so completely hidden by his other arm. Personally I don't think pixel art is a good medium to learn anatomy, I would start sketching to begin that.

Also don't be afraid of using more contrasting colors. The ones you have are a bit bland.
Nevertheless, One Piece is awesome, keep up the good work.