how do i find my serial number on itunes?

Hello there, Fubsic.

The following Knowledge Base article offers just the information you are looking for:

iOS: How to find the serial number, IMEI, MEID, CDN, and ICCID number

http://support.apple.com/kb/ht4061

Particularly:

Getting the information from your computer

Here's how to find the serial number when you are able to connect your device to your computer.

1. Connect your device and open iTunes.
2. Click the Device button in the upper right corner. (If viewing the iTunes Store, click the Library button in the upper-right corner first.) If you don't see your device, choose Hide Sidebar from the View menu.
   
3. Click the Summary tab. The serial number will display as shown:
   

Note: You can choose Edit > Copy to put the serial number on the Clipboard. You can then paste the serial number into an email or webpage (for example, for service or registration).Here's how to find the MEID or IMEI, ICCID, and phone number or CDN.

• With iPhone connected, click Phone Number to rotate through other device information including MEID, IMEI (when applicable), and ICCID.
• With iPad (Cellular models) connected, click Serial Number to rotate through other device information including CDN, MEID, IMEI (when applicable), and ICCID.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.