Pretty Simple. You may want to place the circle on a new layer.
1. Select the Elliptical Marquee Tool (press and hold on the rectangular marquee tool and you will spot it)
2. While holding down the Shift key, drag out your circle with your mouse or track pad, and release the mouse or trackpad key first for that perfect round circle, then release the Shift key.
3 Move your cursor inside the circle selection to position it.
4. In the Edit menu, choose Stroke, and set the thickness and color in the dialog box, click OK when done.
5 Cmd d to deselect and you have your circle.