Start by opening a new image, for example 400 x 400 pixels. Then choose the "Rectangle Tool" and draw an empty rectangle. Like this:

Now you will have created a path to work with.

Ok now it is time to prepare the "special stamp brush". If you have never worked with brush options this will be exciting :)

Find the Brushes tab. If you can't see it try Window -> Brushes.

If you think about it the stamp's border is actually made out of halfs of circles.

Choose Spacing to about 160% and you will get nice circles.

Then set the color of the brush to the color of the background so only half of the circles will actually remain visible on the white backround of a future stamp.