How to get your iPhone's (or iPad's) UDID
When you're developing and testing your apps, as with any software, you'll want to test it thoroughly before you send it out. Your iPhone's UDID is important for setting up test devices.
This process requires iTunes (but not necessarily a Mac)
Plug your phone/tablet into your computer.
Open iTunes (Download iTunes)
Click the little phone symbol that represents your device
In the device information section at the top, observe the device serial field
Click it until it turns it turns into "UDID"