These are instructions on how to find your iPhone's UDID (Unique Device Identifier). You may be asked to provide this to Apple when registering a development device.
Connect your iPhone to a computer via USB and launch iTunes.
Choose your iPhone from the navigation menu at the top left and make sure the Summary section is selected.
Click the serial number of your device to display your UDID. Even though you can't highlight the UDID, you can press Command+C or Control+C to copy the UDID to your clipboard.