Here's what you do.
First, connect to Xbox Live (you have Gold subscription right). Now, on the Halo 3 main menu. Scroll down the the selection, Matchmaking. You will be met by a screen. You can now change game types. Choose from the selection and which one you want. Note that ranked matches are more hardcore and that socials are more laid back. Hit start matchmaking and get ready to start.
Important Notice: Considering that you might be new to the game you might notice that you won't be able to play some of the modes. If this is the case, you need to download the Heroic Map Pack off of Xbox Live Marketplace. You can easily access the Halo 3 Marketplace Page by pressing start on your Xbox controller and pressing the Marketplace button on the menu. Scroll all the way down to find, 'Heroic Map Pack'. It should have 'Free' next to it. Press it. It should take 30 minutes to download on to your Xbox on a good connection. THIS IS REQUIRED TO PLAY MOST GAME TYPES AND MODES.
