Hi there, @gebharp ...
I guess it would depend a little on what you want to show your students. If you are just wanting to show your students the basics of what Canvas is and what features are available, Instructure already has several short tutorials available here in the Community: Guides: Video
If you are wanting to show more specifics, though, such how to access material for your course...or to talk about some of the content within your course, you might have to use something like Free Online Video Sharing With Screencast.com to record your videos and then upload them to Canvas for your students to watch. How do I upload a video using the Rich Content Editor as an instructor? Another option might be if to use Zoom (if you have a paid Zoom license) to record yourself in a Zoom meeting...while sharing your screen. Then, you could download the recording to your computer and upload it to our course.
Another option for you would be Canvas Studio. I'm not sure if your school has purchased Studio from Instructure, but it does allow you to do screen captures right online, and then you can put those into your Canvas courses. How do I record a screen capture video in Canvas Studio?
I hope this information will be of help to you, Paula. Please let Community members know if you have any other questions about this...thanks! Be well, and stay safe!