Before going on camera, consider using Audio & Video Options to preview everything.

Click the Settings (Gear) icon on the upper right:

Under Audio & Video Options you can preview how you look on webcam and make adjustments to your background or sitting position. You can also see your Dial-in information if on phone, or check your mic to ensure it is working or choose a different mic and test your speakers for sound:

Audio/Video Options

You may also do a quick check on your Webcam using the corresponding icon at the bottom of the screen:

Webcam

Or test your speakers, check your mic or switch to telephone using the corresponding icon also at the bottom of the screen:

Speaker Test