It's good for your first, but not without problems.
The tail's shades contrast too much to be dithered like that. Use either only a tiny bit, or none at all.
The right (our right) arm looks thicker than the left. If anything, it should be the opposite since the way it is shaded suggests that the left arm is in front of the right. Howeverm since it's such a small sprite, it should probably just have the two arms about the same.
The pose is also very stiff. In the future, try to use original, more action poses.