Hey, this is certainly better than some of my earlier stuff, so thats good right. Free tips, google image is your friend and so are galleries like Pixel Joint and even The Spriters Resource. Go find references from them and study them to improve your work. Never steal though.
As for critique. Well, the clouds are a bit reddish and the shading makes little to no sense, maybe go ahead and simplify them even more. You are using a very minimalist style, so try and create cloud looking shapes and avoid the shading for now.
Also, more contrast in areas would be good. The darkest brown could do with being darker, and the lightest green, well, brighter. Exaggerating real life range in values (darks to brights) within the pixel art medium can sometimes go a long way.