How do I convert my 2D web game into a multiplayer game using socket.io?

As of right now, I’m working on a 2D web game. I want to make it multiplayer so I can play it with my friends. So far I have only imported socket.io and have gotten stuck figuring out where to start. Could someone please just point me in the right direction? Here is what I have got so far. https://github.com/sherbitfish/web-game