To find the serial number of an iPhone for most models, look in the phone settings. The physical location of the serial number varies on the iPhone model but is usually on the SIM tray or the back of the phone. Alternatively, check the Device tab under Preferences on iTunes.
Go to Settings, choose General and select About to find the phone's serial number, MEID and IMEI. If the serial number is not listed, open iTunes and go to Preferences. Click the Devices tab and mouseover the backup of the device to view the serial number. To open the iPhone SIM tray, insert a paperclip into the small hole located on the SIM card insert and pull the tray out.