how do you make a multiplayer game what do you need to do?

Since you are asking a question about which you know very little(Your question is way to broad) I am going to assume you don''t know what you really want. If you haven''t made a game before make a single player game before making a multiplayer game. You need to understand networking in alot of detail. Check out the article on Gamasutra about Xwing vs. Tie. Alot goes into a multiplayer game. Start with writing a simple network app before you make a multiplayer game.