Editing the entire picture will not always achieve the desired results. For example, the background in this picture is dark and contains undesired colors so we will select everything but the camera.

Selecting specific spots will allow you to eliminate particular issues and edit a picture to perfection.

Some automatic selection tools will create a "marquee" that is very close, too close to the object. Look for a command like Select, Modify, Shrink to reduce the proximity to your object by 1, 2, 3, or 4 pixels. This will leave a small buffer around your object so that your edits don't effect the object as well.