Some quick tips:
Make sure you pay attention to color contrast. Most of the the cowboy dude's sprite is a dark, unsaturated brown where it's kind of hard to make out individual lines. Then you have his extremely bright, saturated skin that draws too much of the viewer's attention.
You can make objects appear larger or smaller through your color choices. That's why the cowboy dude's arms look thicker than his sleeves do.
Read about anti aliasing in pixel art so you have better control over shape and color.
Try to get as much mileage out of your colors as possible. If you can make something look good with three colors, don't add a fourth. Make each shade you use distinct from the rest.
The best advice for learning any visual art is to look at the world around around you. Look at the way light behaves and how light and shadow affect color. Use real objects as references when you draw rather than than trying to intuit or remember what things look like.
E:
Here, maybe this will help:
I did a quick re-do of that sword you were working on. The main thing you should notice about it is how I used darker pixels to smooth out the edges of the blade and make it slowly taper off near the tip. This works because the darker pixels partially blend in with the black background, giving the appearance of a sort of half pixel. This particular kind of anti aliasing won't work as well against a brighter background because the darker pixels will stand out rather than blend in.
It's not perfect but maybe it'll give you a better idea of what to do.
