Instructions
Attach one end of the cat5 cable to the back of your Xbox 360 console. The port is located directly under the bar code and serial number, next to the HDMI port and vent.
Attach the other end of the cat5 cable to a free port on your router or modem.
Go to "System Settings" under the "My Xbox" category on your console.
Select "Network Settings," followed by "Test Xbox LIVE Connection." Provided you haven't changed any of your previous network settings, there should be no issues. If your console doesn't connect to Xbox LIVE, verify everything is plugged in firmly and properly. If it continues to not work, try using another cat5 cable.