Take a screenshot of part of your screen

  • Press Command-Shift-4
  • Move the crosshair pointer to where you want to start the screenshot.
  • Drag to select an area.
  • When you've selected the area you want, release your mouse or trackpad button.
  • Find the screenshot as a .png file on your desktop.

Reference:  https://support.apple.com/en-us/HT201361Apple