To Take Screenshot of Whole Screen:

 If you want to capture whole screen, press the following three keys together

Command Key

Shift Key

3

The screenshot will be captured and the image will be available on your Screen.

To Take Screenshot of Selection

  If you want to capture only a part of your screen, then press the following three keys together

Command Key

Shift Key and

4

A selection image will appear on screen. Just drag it across your desired area , Image will be created on your Mac Screen.