As you mentioned - I forgot the iCloud and the password of my computer.
The password for computer is admin password that is used to login with admin name and its password , if you cant sign in with it reset https://support.apple.com/en-us/HT202860 prefer to reset from recovery mode https://support.apple.com/en-us/HT212190
Once Mac / or computer that is being signed in with admin password , the Apple ID address is difficult to find as you must remember it , say Apple ID address is example1@gmail.com or example2@icloud.com If you forgot your Apple ID email address or phone number - Apple Support it depends how was the account set with secondary authentication or 2FA , do you have all credentials to set like password and three answers for respective questions , date of birth , rescue mail address and its password .
Note : when you perform the steps in recovery mode the Apple ID address is required in upcoming steps pop - ups will appear to set them on Mac desktop .
If you don't have any credentials erase the hard drive Erase and reinstall macOS - Apple Support and reinstall Mac OS and use a different Apple ID to sign in , you have to set up Mac Set up your MacBook Pro - Apple Support
To create a new Apple ID How to create a new Apple ID - Apple Support
Apple support or any body don't know your Apple ID address as account holder have full rights to change it , you must keep the details in your journal .
( a ) If you lose answers for Apple ID provide credit card details that was added in the account .
( b ) if you have lost Apple ID password provide invoice copy of Mac to apple support advisors .
( c ) Contact apple support , they will send a link on mail account , click on link after 24 hours to reset apple id password , however the link is valid for additional 3 hours .
( d ) Support pin is used by apple support advisors to verify the account when you know apple id password Generate a temporary Support PIN - Apple Support
