If you want to share your screen in the Online Classroom, that's no problem at all - just click the Share Screen button on the left hand side of the classroom!
You'll then be asked which screen or tab you would like to share.
You can choose to share either your entire screen (this is really useful if you do have a dual monitor set up), an application you have open and unminimised on your device, or a Chrome tab that you've got open!
If you'd like to share the audio playing on a Chrome tab, for example if you'd like to share a Youtubevideo, you'll just need to click the 'share audio' option at the bottom of the screen share pop, as is shown below.
You can also have a look at this video for more information about how to share your screen!