Forgot id

Have you tried : If you forgot your Apple ID - Apple Support ?

Or, and if you were the original purchaser of the iPad (i.e. it’s not second-hand), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it

Did you buy the iPad and have its receipt ? If you did and have, what did Support say ?

You will either have to find or remember your id, or for Apple to help you will have to get a copy of its purchase receipt.