OK, that means it's a G3 white iBook.
I recommend that you get the CDs that came with that computer from whoever gave it to you as a gift. If they don't have it, then you'll be forced to buy a version of OS X for that computer. I don't recommend getting it illegally as no one would continue to help you here if they were to deduce that.
If you can't afford Tiger, you can always get the previous version (Panther, 10.3.x). It's still quite usable and as far as I know, the only difference between 10.3 and 10.4 is Spotlight and Dashboard. Other than that, there's not much difference in usage. Previous versions of OS X (I recommend 10.2, aka Jaguar, as the minimum OS to purchase) are much cheaper than Tiger at the moment.
Or, you could download a Linux distribution which is completely free, but lacks the ease-of-use that the Macintosh OS is known for. I recommend Ubuntu Linux (
http://ubuntulinux.org) for a desktop user. It tries to mimic the desktop user experience found on the Mac as best as possible, but compared to OS X it's still not quite there. However, if you're willing to, download the ISO image to another computer and burn the image to a blank CD. Make sure that you get the PPC version, as the x86/i386 version of for PCs. You can install this free of charge and legally.
You can also buy a copy of Mac OS 9 which would be significantly cheaper, but support for that has been waning more and more as the years pass.