how to find my serial number in itunes

if you've created a backup, go to preferences (control + comma for windows or Command + comma for mac)

Then, click 'devices'

then, without clicking, hover the mouse thingy over the name of the backup, it should appear.

or try this if you're from US or canada:

Online at My Support Profile

If you used your Apple ID to register or purchase your iPod, you may find the serial number listed in My Support Profile (available in the United States and Canada). Log in with your Apple ID and find your iPod in the "My Products" box on the main page:

Note: You may have more than one Apple ID and not realize it. For more information, visit My Apple ID and click "Find out."