The following instructions for taking a screenshot will work with all Apple computers. (Macbook, Macbook Air, Macbook Pro, iMac, Mac mini)
1.) Press Command + Shift + 4. The pointer changes to a crosshair pointer.
2.) Press the Space bar. The pointer changes to a camera pointer.
3.) Move the camera pointer over a window to highlight it.
4.) Click your mouse or trackpad to take the screenshot of the window.
By default screenshots are saved as a .png file on your desktop.
Original Reference: How to take a screenshot on your Mac