Use Face ID or Touch ID if it is already set up
Use the correct passcode if you remember it
Erase the iPhone with Recovery Mode using a computer
Erase the iPhone with Finder on a Mac
Erase the iPhone with iTunes on a Windows PC or older Mac
Use iCloud Find My to erase the iPhone remotely
Use the Apple Support app or Apple Support website for recovery options
Contact Apple Support with proof of purchase if Activation Lock is involved