Instructions
Purchase an Xbox Live gold membership. Many major game stores and grocery stores sell memberships for one, three and 12 months.
Go to the Xbox 360 code redemption website on your computer and sign into your account. Type in the code on your card and click "Redeem." Your account will automatically be extended by the number of months you purchased.
Turn on your Xbox 360 console and sign into your profile as an alternate means to redeem your code. Press the silver guide button on your controller and navigate to the first menu on your screen. Press "A" on "Redeem Code" and type in the code on your card. Select "Redeem" to automatically extend your gold subscription.
Load "Halo: Reach" to the main menu. Select "Multiplayer" on the main menu. Because your account is now activated with an Xbox Live gold membership, the "Halo: Reach" online features will work as normal.