Hi @rahul
The first thing would be setting up a WebRTC server, the video calls in mattermost need a Janus Gateway server to establish the connection between the peers in Mattermost you can see in the documentation that we provide a docker container with a pre-configured Janus server for your convenience.
if you don't know how to run a docker container I would encourage you to read the Docker docs and then follow the instructions for the mattermost-webrtc docker container or if you prefer you can set up a Janus server by following the instructions here and then once you have that server up and running you can configure Mattermost to use it.
Let me know if that helps you a little bit more