Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in?
https://iforgot.apple.com/password/verify/appleid
See this Apple article: If you forgot your Apple ID password
If you forgot your Apple ID password - Apple Support
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support
See also ⤵️
If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
How to remove Activation Lock
How to remove Activation Lock – Apple Support (UK)
You can also contact Apple support for help, -> Get Support (apple.com)
You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.
If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.
Once Apple receive your proof of ownership, Apple will further review your request and provide an update.