Assuming that you are the original owner of the AppleID that was used to originally activate the iPad, you should be able to recover your AppleID and associated password here:
https://iforgot.apple.com
To recover your AppleID credentials, you will need access to either the primary or “recovery” email addresses that you should have configured when setting-up your AppleID account - or one of the “trusted” phone numbers associated with the account.
When you have recovered your AppleID credentials, you can change your primary email address (and update your account as needed) here:
https://appleid.apple.com
If you update your AppleID account with a new email address, this will change the AppleID username that you use to log-in to Apple services.
If you cannot remember the iPad passcode, you will need to follow this process - which will require AppleID credentials to complete:
https://support.apple.com/en-gb/HT204306