If you want to share your screen in the Online Classroom, that's no problem at all - just click the Screen Share button in the bottom left-hand corner of the classroom!
You'll then be asked which screen or tab you would like to share (shown below on Google Chrome):
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!
To stop sharing your screen, simply click the Stop Sharing button!
If you'd like to share the audio playing on a Chrome tab, for example if you'd like to share a Youtube video, you'll just need to click the 'share audio' option at the bottom of the screen share pop, as is shown below.