Click on the Create New Layer icon in the Layers palette.
Press Option-Delete (PC: Alt-Backspace) to fill it with your Foreground Colour.
Press Command-R (PC:Control-R) to make your rulers visible.
Drag your guides from the top and left rulers to the center of the document.
Guides will snap to horizontal and vertical centers automatically.
If they don't, make sure Snap command is active in the View menu.
You can delete the layer.
Another method is to use the Free Transform bounding box.
Press Command-T (PC: Control-T) and you will find the transform handles in the center of the top and bottom as well as at the sides. You can drag your guides as above to get a centered selection.
Whichever method you use, press the Alt/Option key while dragging the Marquee tool from the center point as marked by your guides.