Making Rectangular, Square, Oval, and Circular Selections in photoshop

Here are some of the options you should learn when using the selection tools.
■ Click at the point where you want the rectangle or ellipse to begin and then
drag in any direction. The selection will grow from that point in the direction
you drag.
■ Hold down the Alt/Option key and click a point, then drag in any direction.
The selection will radiate outward, with that point as its center.
■ To draw a perfect square or circle, click and hold down the Shift key while
you drag. Hold down both the Shift and Alt/Option keys when you first click,
and the selection will radiate from the center point where you clicked.
■ Choose Fixed Aspect Ratio from the Style drop-down list in the Options bar,
Leave the Width and Height values at their default 1, and forget about holding down the Shift key. You’ll draw only perfect squares or circles every time you click and drag when this option is active.
■ Type other values into the Width and Height boxes to create selections with
other proportions. For example, using 8 and 1, respectively, will force the
Marquee tool to create only selections that are eight times as wide as they are tall.

■ Choose Fixed Size from the Style drop-down list in the Options bar and type
in dimensions, in pixels, for your selection. Say you had an image that was
800 × 600 pixels and wanted to grab a 640 × 480 pixel chunk of it. Once
you’ve typed the target dimensions into the Width and Height boxes, clicking
with the Rectangular Marquee produces a selection in that size that you
can drag around the screen to the part of the image where you want it to be.