Press the “PrtScn” button on your keyboard. Then click “Start” → “All Programs” → “Accessories” and open “Paint”. Click the “Paste” button while in Paint. Your screenshot should appear. Remember, the screenshot shows everything that was open on your desktop.
If you want to take a screenshot of an open window only, press the ALT and PrtScn buttons at the same time. Then paste the screenshot into Paint using the procedure explained above.
There are several ways to take a screenshot of all or part of the screen in OS X:
Pressing Shift + Command + 3 takes a screenshot of the whole desktop.
Pressing Shift + Command + 4 takes a screenshot of part of the desktop.
Pressing Shift + Command + 4 + Space will take a screenshot of the current window.