If I want to install Zorin, and I go through the proces, my computer says there is only 8GB, but that is on the USB stick. My grandfather installed it too with the same USB sticks and he had no trouble. We don't know what the problem is.
Try boot up the live USB of Zorin OS. Then System Tools --> Disks
Do you see the the hard disk?
Or when you wan to install choose "Something else" do it pick up the Disk?
Have a look at this pre-installation check list:-