Begin by clicking and holding on the Marquee Button, at the top left of the tools palete. From the list that appears, chose the Eliptical Marquee tool. Then go to your document and make a new layer. To create a perfect cirvle, hold down shift while clicking the tool and dragging your mouse. Create a circle of the size you want, and realease. Then, go to Edit >> Stroke, and select the atributes you want. Then apply it, and deselected the marquee, and you should be all set.