If you want to register your iOS device on the Apple website, then you’ll need to find the serial number. There are a couple of ways to do so.
First, you can plug your iPad into a computer, open iTunes and then find the serial number in the iPad summary screen.
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Or, you could just open your iPad, and then go to the Settings app. In the Settings app, tap on the “general” tab, and click on “about”. The serial number will be down the list, above the WiFi address.