That could be quite difficult by now. Depending on the model, it could be OS8, OS8.5, OS8.6 or various versions of OS9.
It's probably going to come down to the likes of eBay by now, but there may be someone in the Classic OS forum who can advise further.
If you can give a little more information on the iMac, like is it tray or slot-loading for the CD drive, maybe a model no. if it's visible (underneath usually).
Colour - I see you've chosen what looks like a blueberry (or is it Indigo?) one for your avatar, but that still leaves several possible models if it's what you have.
If it's bootable, you can find more info in System Profiler, but obviously you'll need power and keyboard etc to find that out.
Most of those models can run Jaguar or Panther, but the slot-loaders needed a firware update to get to Panther, and that needs OS9 first to install the firmware. And finding retail copies of either of those OSs may not be too easy either!
Check out EveryMac and see if you can find the one you have.