To capture the entire screen

1. Press Shift-Command-5 on your keyboard to see the onscreen capture controls: 
2.  Click Capture Entire Screen in the onscreen controls. Your pointer changes to a camera.
3. Click anywhere on any screen to capture the screen of that display or click Capture in the onscreen controls to capture the screen of every display. 
4. A thumbnail of the screenshot briefly appears in the corner of your screen. You can then find the screenshot on your desktop.

To capture a window  

1. Press Shift-Command-5 to see the onscreen capture controls. 
2. Click Capture Selected Window in the onscreen controls. Your pointer changes to a camera.
3. Click a window to capture that window.
4. To exclude the window's shadow from the screenshot, press and hold the Option key while you click.
5. A thumbnail of the screenshot briefly appears in the corner of your screen. You can interact with the thumbnail to edit the screenshot, move it, or take other actions. Or wait for the screenshot to appear on your desktop. 

To capture a selected portion of the screen 

1. Press Shift-Command-5 to see the onscreen capture controls. 
2. Click Capture Selected Portion in the onscreen controls. 
3. Drag to select an area of the screen to capture. To move the entire selection, drag from within the selection. 
4. Click Capture in the onscreen controls.
5. A thumbnail of the screenshot briefly appears in the corner of your screen. You can interact with the thumbnail to edit the screenshot, move it, or take other actions. Or wait for the screenshot to appear on your desktop.