To begin this tutorial, start a new document (I used 450 x 450).
Select the 'Background Layer' in the Layers Palette, then hit 'Ctrl/Command + R' to show the Ruler Bar's (If not already visible)
Then drag a Horizontal and Vertical Guide from the Ruler Bar to meet at the center point of the new document.
TIP: Make sure 'View/Snap' is turned on. Then drag the guides
SLOWLY and you will notice that the guide will automatically find the center point of the document.
(Applies only to the background layer and individual layers)
Then with the 'Ellipse Tool (U)' and 'Paths' turned on in the 'Options Bar', draw a Path and position it to the center of your document, as I have captured to the right.
(I also dragged an extra Guide to the top of the Path)