At the main menu, look down. You will find 4 games to play. click on one and click “Join a Game”. Click on a game and click “Join” to join the game. If you want to start a game yourself, click on “Create a Game”. You will be able to select how many players can join your server, if only your steam friends can join the game with you, and the map you want to play. When you are finished, click “Create”. A server will be made available to join through the “Join a Game” menu that other players can join, unless the player limit is one or the server is full.