I tried submitting this but it got rejected, so how can i improve it?

AAing would help this piece a lot, especially around the eyes and outer circle, both of which are rather jagged at the moment.  The light source is a little odd and doesn't seed to effect the character much.  I'd suggest moving the lightsource to the top left or right, instead of the side.  More contrast between the standard color and the highlight color might help as well.  As far as the design of the character goes, this is about as bland as they come.  It's really nothing more than a smiley with stubby feet, even kirby is more intricate than that.  Adding hair, actual shoes, arms, open mouths, hats, SOMETHING would help immensely.