To take a screen capture simply hold the Shift, Command and 3 key simultaneously. A thumbnail will appear in the corner of your screen which you may click to edit the capture. If not, it will automatically save to your desktop.
iPhone or Android
Taking a screen capture on these devices is quite similar. This may also differ depending on the model you may have. The latest iPhone models operate screen captures by holding the Lock/Unlock with the button to increase volume.
Android devices are not as uniform, therefore screen capture commands may differ. Typically, you will need to hold the Lock/Unlock and home button.