When you enter Video Conference for the first time you will be prompted by web browser to allow access for you webcam and microphone. You can set your preferences there - which to allow/disallow and which device to use. You can go back to those settings at any time.
Once you are in Video Conference you will have options to switch on/off your microphone and camera. You do this by hovering mouse over your video field and in the top right corner you will see two icons - one for video, other for microphone - where you would allow or disallow them.
For low internet connections it is wise to turn off camera as it will improve video call quality.