Are you playing with the account you're posting with? If so, you're a Silver (Free) account, and you will need to purchase a Gold membership before playing online on Xbox LIVE.
I would like to mention that if your Xbox 360 is new, you should have Xbox LIVE 1-Month Trials that will automatically activate when you make a new Xbox LIVE Account from the Dashboard. When you turn on the Xbox 360 and you are at the Dashboard, sign out
of any profile you may be signed in on. Then make a new profile by pressing the
Guide button, and press Create Profile. After you name the profile and choose a basic avatar, you will have the option to press
Join Xbox LIVE. Press that and follow the directions on-screen. At some time while making your account, you should be prompted if you would like to use a 1-Month Gold Trial.
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