If you mean create a lobby/game, go to the main menu,then look at the right-south part of your screen, it should say if your friends are playing L4D, push the blue X button (360), then push X once more to create a lobby, now choose a lobby: Campaign, VS, Survival.Pick one,and keep it on Friends-Only, choose your level, create lobby ,choose character, then change to public-game, receive wanted amount of players and then start game.