[Myomoto]

Pillow shade like that again and I'll strangle you! D:
And if you don't know what pillow shading means, look it up, and I'd suggest you start googling for som pixel art tutorials, because thaht's the best way for you to learn how to create a circle. (your green center looks insanely jagged).
And did you use a gradient fill for the center or something?

Again, my only suggestion is just to study a little bit of theory.