How do I wipe my hard drive clean so I can sell my iMac G3?

You need to boot to the Software Install CD-ROM that came with the iMac. To do so boot the computer off the CD-ROM by holding down the C key. Once it boots up open the Utilities folder displayed in the open CD-ROM window. Open Drive Setup. Open the Functions menu. Select the option labeled Options. In that window select the Zero All Data option checkbox. Open the initialization menu. Select initialization options. You want to make sure the iMac formats its hard drive in HFS+ or Mac OS Extended. Once you've selected all the initialization options click the Initialize button. It will then format the hard drive & check it for bad sectors. Then reinstall the Mac OS. Then it should be ready to sell.