Would you be able to recommend some basics for learning multiplayer development?

Hey guys,
Currently, I am able to do the following using Unity:
First-person game (Character and simple scene)
A simple 2D game (with a character and a scene)
how to make a user interface
What is C#? How do I program in it?
Multiplayer games (I mean online multiplayer) are really what I want to learn.
I'll be motivated to learn more if that happens.
So what basics do you think I should have before tackling that? And especially what tutorials/documentations could you recommend?
What are the alternatives to photon? It's hard for me to know where to begin.

I appreciate your help.

 

i've collected few good ones here (for example the last one for unitys own multiplayer system)
but check those c# tutorials too.
https://gist.github.com/unitycoder/b0903bed09528c38c03b59e05d4e1138

but probably not good idea to go into multiplayer rightaway, if you havent coded anything yet.

unity learn has basic c# tutorials too
https://learn.unity.com/search?k=["tag:5814655a090915001868ebec"]

and for making ui
https://learn.unity.com/search?k=["tag:5818e455090915002eeb1b8a"]

but often you find better, up to date tutorials in youtube..

 

There's an official FPS tutorial project tutorial on here but it's at least 3 years out of date.