How do I contact my administrator and how do I know who “my” administrator is?

Well, since your iPhone has an administrator (unlike most iPhones), your administrator is some IT person at the organization governing your iPhone - presumably an employer or educational institution.

How to fix the device management

Was the price “too good to be true”? Now you understand why.

Return it if you can.

Hello~ I would suggest you get your parents involved since you are a minor.

~Katana-San~