how to boot an imac after installing new clean drive.

It's formatted incorrectly to use it with Mac OS. It must be formatted as Mac OS Extended (Journaled) GUID partition. They way it's formatted is for MS Windows so if you want to use it as a Windows box then you are on the wrong forum! If the computer is a 2012 or later you can attempt to do an Internet Recovery once it is formatted for Mac OS. If it's a 2011 or earlier then you will need to buy a Snow Leopard (10.6.x) DVD which are still available on E-Bay. No you cannot download them from the Internet, you actually have to buy one. Once you have the DVD then you can install Mac OS 10.6.x on it and then upgrade it. As we have no idea what year it's from you will have to research that using the serial number which is likely on the base of the stand. You can use https://everymac.com/ultimate-mac-lookup/

Good luck with your "find."

Try holding down Command+Option+R right after you press the power button. You need to do this with a wired keyboard, but this will put you into internet recovery. There should be an option to reinstall MacOS, but you first may need to use disk utility to format the drive to a format compatible with MacOS or OS X. Hope this helps!