Step1: Start iTunes
Step2: Connect your iPhone, iPod Touch or iPad and wait until iTunes list it under "Devices".
Step3: Select the device
Step4: Click on the 'Serial Number' on the Summary tab. The 'Serial Number' display will change to Identifier (UDID)
Step5: Copy and paste it into an email. CTRL + C (on Windows) or AppleKey + C (on Mac)
Step6: Check that the code is 40 characters long.