Mac Pro wiped clean, will not install OS from USB

Got an older Mac Pro from a public schools surplus. It was wiped completely clean before I bought it. I have a High Sierra OS on a USB drive that I have tried to boot the computer from. I try to reinstall the OS and it says that I need a firmware upgrade before I can install. I have tried "holding the power button until it blinks or a I hear a long tone," I have held every sequence of keys I can read about to get the upgrade to occur. I am on the chat with Apple supprt currently and I am not getting any closer to having it installed. I have done the compatibility check, this machine will run HS so I am not sure what is going on. I am NOT a mac guru so I need specific instructions please, if you have an answer. Thanks in advance.