Hi, appleibookg3 -
Welcome to Apple's Discussions.
I have an install disc for Os 9.0....
If that disk is a full-install CD, either model-specific to that machine or a retail one (the retail OS 9 Install disks have a white label with a large gold 9), then all you should need to do is boot to the CD and run the installer (Mac OS Install).
The installer will not run in OSX - that is, while the machine is booted to OSX. Hence the need to boot to the CD, so the machine is being run by OS 9 on the CD.
If you're not familiar with boting to a CD - with the CD in the drive, restart or boot. Immediately press the C key, keep it held down until you get the Welcome to Mac OS 9 screen.
Note - it takes significantly longer to boot to a CD than it does to boot to an OS on the hard drive.
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If the previous owner re-initialized the drive before installing OSX, when you boot to the OS 9 Install CD you may not see the icon of the hard drive on the desktop. That issue is described in this Apple KBase article -
Article #106849 - Disk Is Available in Mac OS X But Not in Mac OS 9
That article mentions one possibility of recovering from that situation, by using Drive Setup to re-install OS 9 drivers on a drive which once had them.
Unfortunately that rarely works, and it then becomes necessary to re-initialize the drive again, this time ensuring the option to install OS 9 drivers is selected (if using OSX's Disk Utility to do the work). Re-initializing the drive will erase everything on it, so it is necessary to archive off anything you do not want to lose before redoing the drive.