In order for the participants to see and hear you, use a webcam and a microphone (both are usually built into every laptop). When you open Video.Taxi Studio for the first time, you need to allow your browser to access the webcam and microphone in a small pop-up window.
Now select the camera and microphone you want to use (if you have external devices connected, you can select them here) and you're ready to go.
If you still have problems, check your browser settings (for Chrome, for example, the three dots in the upper right corner) for website permissions, and enable camera and micro permissions.
For Apple devices, you must have enabled camera and micro permissions for your browser under System Preferences - Security & Privacy - Privacy.