The first time you make a video call the internet browser will request permission to use your camera and microphone. You only have to do this once, your browser will remember your permission for future conversations.
The way you can give permission differs between browsers. Below some examples are shown:
With Internet Explorer you need to install a Plugin.
Unfortunately Safari does not yet support the technology used to make video calls, therefore video calls do not work in Safari.
If your browser does not automatically show the request to use the camera, you can click on the icon of the camera in the top of the screen.