That is a huge project. You'll have to do/write...
- Graphics design. In 3D, even. Start at the Unity manual[^] for this. I don't know how fancy you'd like your game to be, but there's a lot of Graphics material to go through.
- The game client code. Again the Unity manual can be your starting point. Though you'll also have to write the networking code to communicate with the game server. Look at the
Socket and related C# classes in the documentation.
- The game server code. After all it's multiplayer. You need a central application that handles all data that comes from the clients and sends the relevant game data to the relevant clients.
Again, that is huge. This is going to be a lot of work. Most popular video games are therefore written by a whole team of people. Especially if you don't have a lot of experience with it, you'll first need to learn about everything related to Unity, graphics, and the networking code. For that you'll need to start with smaller, much simpler, projects/games first, before you can take on something big like this.