Create a new layer (ctrl+n)
Name: Button Main

Draw a circular selection, 18 p x 18 p

Fill with color of Your choise

Select the right half of the circle

Use ( ctrl+alt ) + the right arrow key on keyboard to "drag" out the button to the width You would like to have.
Should look like this:

Know we have to get the button little color.

New Layer ( ctrl+shift+n )
Name: Button Color

Hide Button Main layer by clicking the Eye in the Layer Palette.

( ctrl+click ) layer - Button Main
Fill with any color, gradient or texture.

FINISHED!!!

I often Use gradient.
Start fill from top to bottom with Light color to Darker color.
Gets a little rounded effect, like this:

Looking pretty sharp to me, :o)