Capture the entire screen
Press Shift-Command-3, then find
the screenshot as a .png file on your desktop.
Capture a window
1. Press Shift-Command-4.
2. Press the Space bar. The pointer changes to a camera 
.
3.
Click a window to capture that window.
To exclude the window's shadow from the screenshot, press and
hold the Option (⌥) key while you click.
4. Find the screenshot as a .png file on your desktop.
Capture a selected portion of the screen
1. Press Shift-Command-4.
2. Drag to select the area of the screen to capture. To move the
entire selection, press and hold Space bar while dragging.
3. After you release your mouse or trackpad button, find the
screenshot as a .png file on your desktop.
Capture a menu
1. Click the menu to reveal its contents.
2. Press Shift-Command-4.
a. Drag to select the area of the menu to capture, then release
your mouse or trackpad button to capture that area.
b. Or press Space bar to change the pointer to a camera 
, then click the menu to capture it.
3. Find the screenshot as a .png file on your desktop.