Ho do I register a new iPad?

There isn't a need to register (except for AppleCare in some cases when purchased separately). Warranty is tied to the product serial number.

You can check on warranty (and AppleCare coverage) via: Check Your Service and Support Coverage - Apple Support

Also see: My Support - Official Apple Support

If hou bought it from Apple, it’s registered automatically. If you bought it elsewhere, registration should be automatic once you sign into iCloud.