How to make a Multiplayer Game?

I understand that this is incredibly time consuming and difficult. I would like to learn how to create a multiplayer fps game in Unity. I need a place to start. I would like any recommendations on where to go to learn how(Mainly coding and GUI). I have made a simple tower defense game. I do have around 300+ hours of experience with Unity, but this is currently beyond me. Any help is appreciated.

You can use photon for multiplayer. I have used it into my game and it works like a charm. Here is some samples on the official site.

I am in the middle of making a multiplayer game myself, here is the few websites that helped me greatly:
http://vimeo.com/33996023#

Hey,

My Company, Nextpeer, provides an SDK with which you can easily create cross-platform: iOS, Android and Unity multiplayer games.
Check us out at:
https://www.nextpeer.com/how-to-make-a-basic-integration-with-nextpeer/

Disclaimer: I work for Nextpeer.

I haven’t tried programming a multiplayer game in Unity so far, but your best point to start is learning how to use sockets in c# - they’re the bread and butter of any multiplayer game.
There are tons of places where you can learn about sockets :).

You can try AppWarp. AppWarp is a realtime multiplayer game engine. You can either use cloud based appwarp service or use AppWarpS2 which is a on-premise solution for appwarp.