Alright, to start this long path I’d recommend getting the basics of the engine and game development down first, so I’ll start with recommending more beginner resources. As Predalien mentioned it’s going to be imperative for you to know the engine itself well before you can start working with networking, otherwise you’ll get way too overwhelmed.
The beginner resources are a learning path that gets you accustomed to the engine with some tutorials, then gets you to practice a tiny bit of C++. You will need to know C++ to make an MMO, so start practicing it daily. Dedicated servers and server architecture in general is significantly harder to work with otherwise. Finally in the beginner camp there’s a tutorial series for a singleplayer RPG. You’ll need to know how to handle a singleplayer game before you can move on to multiplayer, but don’t expect what you make in that tutorial to be easily ported to multiplayer after the fact.
For the Intermediate resources you’ll get into networking, setting up dedicated servers, and manipulating players in multiplayer. This is when whatever you’ve made up until now will probably be tossed in favor of rebuilding with multiplayer in mind. If you still need some structure there is an MMO tutorial here (that’s rather dated so you will need to know everything above to use it effectively)
Good luck, this journey is a tough one 99% of indie developers can never make it through and there’s a million ways to progress. I know doing the small simple tutorials at the top aren’t fun, but they are necessary to learn. You’ll still need to find more resources in the middle of all of this constantly.
Beginner Resources:
Intermediate Resources:
