clean system install


I have just acquired an Imac 9.1. It comes with Leopard 10.5 installed. From research I believe this Imac can run El Capitan as a max. I have been trying to update system by system till I reach El Capitan with no luck, in fact I can’t even get started. Snow leopard, for which I have Imac specific discs will not install. After booting from system disc I get the message Cannot install on this machine or similar. I used these discs to load on to my wifes Imac 7.1 with no trouble some time ago and have now updated to Lion on that machine. The HD disc on the newer Imac is formatted correctly but may have been partitioned.

Question: There is nothing on the new Imac I need to keep as it came from elsewhere. If I wipe the disc could i just install El Capitan straight off?
Is this what they call a clean install and is there any reason it wouldn’t work? I have El Capitan which I downloaded from the apple site and I‘ll make a bootable usb drive.
If so could somebody talk me through wiping and preparing the disk, I presume using disk utiilty from the boot disc. The Imac has adequate RAM.
Thanks in anticipation