Instructions
Connect your ethernet port into both your router and the Xbox. If you purchased a Comcast modem, you can connect directly into the modem instead.
Connect the modem or router to the cable outlet. If you are using a router, you can plug several Internet connections into the router; however, you will only be able to connect the Xbox to the modem's single input.
Turn on the console. Scroll down and select "My Xbox." Scroll right and select "System Settings." Select "Network Settings." Select the option, "Test Xbox LIVE Connection." When prompted, select "Yes" to test the connection.
Press "B" until you reach the home menu again. Select "My Xbox" and select your avatar. From the menu, select "Join Xbox LIVE." Follow the on-screen instructions to complete the installation process.