You can take a screenshot on Mac OS X with following simple steps.
Take a screen shot of the whole screen
1. On the keyboard press Command-Shift-3
2. The screen shot is taken, and it is saved as a file on the Desktop
Take a screen shot of part of the screen
1. On keyboard press Command-Shift-4, then drag the cross-hair pointer to select the area to capture.
2. Continue to press the mouse button, release the keys.
3. When you are ready to take a screen shot, release the mouse button.
4. The screen shot is taken and it is saved as a file on the Desktop.