In order to share your screen, click on the green "Share" button at the bottom of the Zoom screen.
From there, a screen will pop up giving you options on which window or desktop to share.
After clicking on the window you will be sharing, be sure to click on both of the checkboxes on the bottom left of that window to ensure all sound and video clips on your screen will be seen and heard from your participants.
After those boxes are selected, you can press the share button on the right side of the window.
You can then select Stop Share (in red) at the top of your screen at any time.
Here is a video to show you how to start and stop your Screen Share: