How to Use the Rectangular Marquee Tool in Photoshop CS6
In Photoshop CS6 the Rectangular Marquee tool creates rectangular, including square, selections. Use this tool when you want to zero in on an image, plucking it out of a larger background to provide a better focal point.
Follow these steps to make a selection with the Rectangular Marquee tool:
Select the Rectangular Marquee tool from the Tools panel.
You can also use the keyboard shortcut — press the M key.
Click and drag from one corner of the area that you want to select to the opposite corner.
While you drag, the selection marquee appears. The marquee follows the movement of your mouse cursor (a crosshair or plus sign icon).
Release your mouse button.
You now have a full-fledged rectangular selection.
If you want to create a perfect square, hold down the Shift key after you begin dragging. When you have your desired selection, release the mouse button and then the Shift key.
If you want to drag your selection from the center outward, rather than from corner to corner, hold down the Alt (Option on the Mac) key after you begin dragging. When you have your desired selection, release your mouse button and then release the Alt (Option on the Mac) key.