Working with selections is one of the most common Photoshop tasks, so you'd better bone up on your selection shortcuts. Holding SHIFT with an existing selection present will add to it. Holding ALT will subtract from it. Holding SHIFT + ALT will leave the intersection of two selections. Holding ALT when starting a fresh selection will center its origin point. Holding SHIFT when starting a fresh selection will constrain its proportions to equal length, producing a perfect square or circle.