For full screen screenshot:
To take a screenshot, press and hold these three keys together: Shift, Command, and #3. If you see a thumbnail in the corner of your screen, click it to edit the screenshot. Or wait for the screenshot to save to your desktop.
For a portion of the screen:
Press and hold these three keys together: Shift, Command, and 4. Drag the crosshair to select the area of the screen to capture. To move the selection, press and hold Space bar while dragging. To cancel taking the screenshot, press the Esc (Escape) key. To take the screenshot, release your mouse or trackpad button.