What are the basics to making a game multiplayer?

What do I need to know? I am making a 3D game. It does not have to be to complex. The first version of my game will store data in the local storage but this will later be replaced. I would like a login screen. please help.
One thing I also need to know is how to set up the controls and how to get the game to spawn an entity for the player to control when he enters the game.

Multiplayer game making - is never basic in first place.
I recommend starting from trying to write server using node.js + sockjs. Server-Client architecture, where server does all game logic, and clients just render stuff and listen to what server tells them - is the way to go.

So in few steps:

  1. Install node.js
  2. npm install sockjs
  3. make small page that will be able to communicate with server, sockjs has tutorials (do this not in playcanvas)
  4. then using playcanvas do same as #3.
  5. implement concept of “client” on the server, and learn how to send out game data in constant periods to clients

Learn slowly, but steadily. Understand bits you do, don’t copy-paste code, if something you want to do is way beyond your understanding, that means you need to slow down, get simpler targets, and iterate over.
Networking - is hard, very hard.