A hint is provided in the lock screen.
Click on this link to learn 'How to remove Activation Lock' --> https://support.apple.com/HT201441
"If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> https://support.apple.com/HT201354
Reset device:
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does provide details as to what is acceptable proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).
Apple recommends you "Keep your receipt after you buy an Apple device" --> https://support.apple.com/HT202711