The latest system the G3 iBooks can run is OS 10.4.11. That's the Mac system known as Tiger. You can buy it used on DVD from places like Ebay. You install it by putting the DVD into your computer's slot and following the instructions. Even though it's an old OS, it's not particularly cheap -- since there are plenty of people like yourself who are using old computers that can use it. You could probably get OS 10.2 or 10.3 cheaper -- and they actually might run a bit faster on an older computer like this than 10.4 would.