I can't see, I can't hear

Have you allowed the website to access the camera and microphone?
Click on the small camera icon in the URL bar in the upper right corner for Chrome and in the upper left corner for FireFox and allow access.
Test your video and audio for jitsi video chat: https://janus.conf.meetecho.com/echotest.html
(pressing start creates an echo-chamber (you hear yourself slightly delayed))