You will see a red "share screen" button in Zoom. When you are ready, click on the button and choose the item / app on your desktop that you want to share. It will appear in the middle of the screen and participants will be relegated to smaller windows (you will still be able to see them, and they you). When you start sharing the screen, the button will change to "stop sharing". Just click on that to return to the original layout.
Why don't you just make a "practice" call with a friend to familiarize yourself with everything?