Welcome to pixelation and getting hammered
Skeddles is quite right, a great place to start would be the tutorials. There is something energetic and fun about your work already, but working on your fundamentals and learning more about how pixel art specifically works will be really positive influences on you I think.
Hopefully I won't re-tread skeddles' points too much, but if I do, well...oh well:
1 - Too much reliance on coloring-book style execution! This is very common when you are just starting out. The best thing you can do is work really hard at trying to see your art as a collection of volumes, rather than lines that contain colors. For example, try drawing this character using NO outlines and only three shades of gray (NO colors) and see what you get out of it! This should highlight a lot of the weaknesses that are introduced by approaching your art like a coloring book.
2 - Another classic beginner's error is this tendency to (a la coloring books again) compartmentalize all your colors. In your case, Yellow is for skin, Gray is for armor, Blue is for pants, Red is for cross. Pixel art is the art of making every color and every pixel really count; but even traditional paint-on-canvas painters will only really use about 6 or 8 basic colors as they work. For example, in this particular piece, you'll find that your dark gray and dark blue are fairly similar colors, and that the brightest shade of yellow and red is practically identical. For the sake of efficiency it is a good idea to re-use colors as much as you can, but you'll find that as you mix your colors more, you'll get a more cohesive piece of artwork as well.
3 - No real lightsource or consideration of the character as a 3d object in space. This is something that will be made more obvious if you attempt to do a simple, grayscale painting of this character with no outlines.
Again, welcome to pixelation, and enjoy your journey