How to convert a client side HTML5 game to a multi-player web game?

I made a html5 tank game, the player can control the tank and shoot enemies, it's a pure client side game and has no server. Now I want to convert it to a multi-players web game that each player can play it at his/her own computer(browser), connected through Internet, every player share the same stage and each player can see the other player's action/move/shoot...etc, where should I begin?

I have no game development background, and this is my first little game, I know java and js well, and also know some server side knowledge like SQL and NoSQL. Please shed some light on this, such as which frameworks available to use, the client-server side architecture, or just some thought would be OK.

Every input is appreciated.