See this Apple article: Erase iPhone
Erase iPhone – Apple Support (UK)
Have you forgotten your passcode?, See this Apple article: If you forgot your iPhone passcode
If you forgot your iPhone passcode - Apple Support
Have you forgotten your password? -> 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
You can use a Windows PC, if you have one, or ask a friend or relative to use their computer.
If you want to visit an Apple Store, call Apple in your region see: Contact Apple for support and service
Contact Apple for support and service - Apple Support
You will first have to make an appointment due to the pandemic.