There are 3 ways on how you can take screen shots in Mac.
1. Command + Shift + 3
Take a screenshot of your whole Desktop and automatically saved on the desktop.
2. Command + Shift + 4
This will enables you to select a screenshot of an area and will automatically saved on the desktop.
3. Command + Shift + 4 + Space Bar
Take a screenshot of a window application and will automatically saved on the desktop. This step is most recommended especially if you are making a step by step tutorials.