Here is how to take a screenshot on an iMac or a portion of it (or any other Mac running OSX)

The screen shot feature is one of many features built directly in to OSX.

Capturing the Whole Screen

Whilst holding down [command] and  [shift] press [3]

Capturing a portion of the screen

Whilst holding down [command] and  [shift] press [4]

You will see the mouse pointer turns in to a cross hair.

Simply Left click and drag the area you wish to create a screen shot from and release.

Using both of these methods you will find a new file on your desktop with the newly create Screen Shot you have created.