Norton 360 requires me to login as admin, but I have never done this and do not know the admin password. How can I find it?

Welcome to the Apple Support Communities.

Delete Norton immediately!

Mac computers do not need any sort of anti-virus, cleaners, VPNs, etc. The only thing they are good at is creating false marketing promises and taking consumers' money. They are all scams. In fact, anti-virus and other cleaners cause performance issues, security issues, and make macOS appear buggy. Your Mac is far worse with these types of Apps installed.

Mac computers have protection built-in, which is the best protection for your Mac. For specifics on macOS Security, see: macOS - Security - Apple and Protecting against malware in macOS - Apple Support. Mac computers also have built-in storage recommendations: Optimize storage space on your Mac - Apple Support. You can even use the built-in macOS Firewall: Block connections to your Mac with a firewall - Apple Support. Other than that, all you should do is restart your Mac approximately once a week and keep macOS up to date: Update macOS on Mac - Apple Support. 

Jack

If you set up the MBP, you are "the admin". Did since you are using the system, you apparently have that "admin password".