Hi Grace Bayers,
Welcome to the Support Communities!
The information below may be able to answer your questions about how to use an external display with your MacBook Air. Click on the link for more detailed information and screen shots:
OS X: How to use multiple displays with your Mac in Mountain Lion and earlier
http://support.apple.com/kb/HT5019
Video mirroring
Mirroring displays the same image on each display.
- Make sure your additional display is properly connected and powered on.
- From the Apple () menu, choose System Preferences. or Press Command-F2 to detect displays
- From the View menu, choose Displays.
- Click the Arrangement tab.
- Enable (check) "Mirror Displays" in the bottom-left corner.
In the example below. the blue boxes represent each display that is connected. The white bar at the top of both boxes represents your menu bar and is shown on both displays when they are showing the same information.
I hope this information helps ....
- Judy