To share your screen, select the green "Share screen" option at the bottom of your Zoom screen. 

A dialogue box will appear asking you what you would like to share--you entire screen (which will follow you wherever you click) or a specific file or app (which will remain visible to your participants even if you navigate to a different page). 

Note the options at the bottom to share computer sound (necessary if you plan to share video or audio) and to optimize screen sharing for video clip. 

Sharing a "Whiteboard" brings up a blank screen on which you and your students can write and annotate. 

Clicking advanced options at the top of the screen sharing dialogue box will bring up some additional options--PowerPoint as a virtual background; a portion of the screen; computer sound only; or content from a second camera.