1, Make a circular selection on a new layer.
Choose your colors. Set the
foreground color and set the
background color The background color
should be the darker color of the foreground color. If my foreground color is red then the background color
should be a very dark red color.

2, The settings for the gradient tool should be
radial for the type of
gradient and
foreground to background for the gradient.
Select the
gradient tool and apply a graident starting from the
upper right corner of the
circular selection. Hold the
SHIFT key while you drag the gradient to the lower left side of the
circular selection.

3, Apply a drop shadow or other effects to complete the 3D appearance.