Well, it's not too bad, but it could be better.
Pikachu's shape really needs to be fixed, especially because on the right, it goes from body to head in one straight line. Try to make its head a bit rounder and seperate from the body.
Also, you didn't really shade it properly. You just made the upper left corner all highlighted and the bottom right corner all shaded. That isn't the case. Look at these
Pikachu[/ULR] [URL="http://4.bulbagarden.net/graphics/pokegra/normal/front/025-3.png"]sprites. Notice how there is highlighting in a circle on Pikachu's head and there is shading under its chin. The shape of its body creates highlights and shadows, but because you did such a poor job, it still looks like a flat piece of paper.
Take out a metal cylinder and put it under a light. Notice how the sides are darker and it's lighter in the middle. Same basic idea with the ears and right arms. Whatever is at an angle where light doesn't hit it, there is a shadow, or if something is blocking the light (like the ear over the tail). There isn't anything blocking the right arm, so it shouldn't be all in shadow.
This is hard to explain without visual aid, so I suggest you go read the Stickies. There are also some very helpful shading tutorials on the internet you should keep your eyes open for- they can really help.
Overall, it's okay, but it really could be better. Just keep practicing!
~Psychic