Hi! I AM a super newbie wanna B coder. Can any of Uuuu Rock Star Coders tell me how to via the Mac OS terminal reset the "root administrator" for my Macs?

I have a 2009 iMac that is running on High Sierra. I cannot change any preferences on the computer or keychain. I can’t download a new application…nothing. I didn’t set up the administror either. I tried just Control-Option-R-P to restore the entire computer. It would not work. I just had a new hard drive put into my iMac computer. My last hard drive was stolen out of my iMac…That’s another story. Any ideas rock stars? I am learning as I am going. i have all Apple products by the way.