1. Click the Share Screen button located in your meeting controls on the bottom of the Zoom Window.
  3. Select the screen you want to share. You can also choose an individual application that is already open on your computer, the desktop, a whiteboard, or an iPhone/iPad.
  4. (Optional) Enable these features:
    • Check Share Computer Sound: If you check this option, any sound played by your computer will be shared in the meeting.
    • Check Optimize for full screen video clip: Check this if you will be sharing a video clip in full screen mode. Do not check this otherwise, as it may cause the shared screen to be blurry.
  5. Click Share.
    • Zoom will automatically switch to full screen to optimize the shared screen view. To exit full-screen, click Exit Full Screen in the top-right corner or press the Esc key. 
    • To disable automatic full screen when viewing a shared screen, disable this option in your desktop client settings: Enter full screen automatically when a participant shares screen.


Source: https://support.zoom.us/hc/en-us/articles/201362153-Sharing-your-screen