What part do you need advice on?
It makes it easier for us if we dont have to write a book and guess on what you know and don’t know.
I would spend some time planning out your game on paper. There are no real rules to follow just get your ideas down and organized and keep track of things you cant answer. Then come back here with specific questions and we can be more helpful.
Your first pass you may not come up with questions, because you know how it should work at a high level. If this is the case, try breaking parts of your idea down a little further into smaller parts that you may or may not be able to answer.
From your key words:
Godot Multiplayer
Steam multiplayer godot plugin
Publishing on steam costs money, and you may not be able to do so legally because of your age without your your parents help. But here is a discussion on this subject.