Sign in at xbox.com. Click Manage Profile in the drop down bown beside your gamerpic. Then click on Registration and Repair at the bottom of the list of options. If your console is already registered it will be listed and you can click "Request a repair" if it's under warranty. Follow the steps from there.
If you do have call xbox tech support or whatever, just start hitting zero until the automated voice system tells you he can't understand and will put you through to someone.