Hi,
You need to get OS 9 to start with. The link I provided above for OS 9 is for a cheap iMac install disk that should be good for your iMac. Being an iMac install disk rather than a retail one it will only work on the iMac G3 but it will include the bonus software that was supplied with your iMac (Appleworks, some games etc.).
I recommend when you get the disk(s) that you boot from them (insert them when you turn the machine on and hold the 'C' key down (C for CD get it!)) then do an 'Erase and Install' this will clear everything off the Hard Disk and restore you to how the machine was sold.
Once you are there you are good to go. You won't be getting a very modern Mac experience as OS 9 is the equivalent of Win98 so hardly cutting edge.
You can, however, upgrade to OS X at a later stage. There are loads of guides on these pages on how to do this - roughly:
- Upgrade your RAM - your iMac will have a PC100 64Mb 168pin DIMM in one slot. There is another identical slot - just stick a 512Mb in that.
- Upgrade your firmware. (You must be booted in OS9.1 or higher to do this)
- Consider upgrading your HDD - any standard IDE drive up to 127Gb will do OS X takes about 2Gb for a base install.
- Obtain a copy of OS X (Panther (10.3.9) is the highest you can technically go to).
Post back if you need any further guidance.
regards,p>mrtotes