If you purchased the physical AppleCare Plan box from the store, these instructions will help you find the registration number.
If you purchased AppleCare while buying an Apple product, then it is auto-registered to that Serial Number. You can check the registration by going here and entering your serial number, IMEI, or MEID for your product. You should get an email with the registration number, but if you do not, you can contact Apple to get it.
Message was edited by: Jay-Ray