I know this is already stated, but “I want to make a multi-player game” is a very broad statement. And this is one reason why you’re not going to get specific answers here. Before any one can even start guiding you, we need to know what kind of multi-player game you want.
Word with Friends (two player, turn based) will have a radically different answer than World of Warcraft (massive multi-player, real time). A card game might just have four seats. Are you going to want 4 players on their own screens and in the same room? Do you want all for players on a single screen like a TV and their screens are just controllers? Or does your card game need to support multiple games of our people each (i.e. you need a lobby to gather players to a table).
Every one of these examples has drastically different responses as to where to get started. The only point we can tell you start is to define your game better. Narrow down the scope.
Rob