Instructions
Use a computer that is hooked up to the same router as the one you want to use your Xbox on, open up a browser, and type in your router address.
Click "LAN Settings" and verify that DHCP (Dynamic Host Configuration Protocol) is turned on. By default, it should be. If it is not turned on, turn it on.
Log out of the router.
Turn on your Xbox and go to the Network Settings tab. Select your wireless network. Enter your wireless network passphrase. Select "Yes" if you are prompted to update your console software.
Test your Xbox Live connection.