As Gromit says, if your system support Leopard then wait a week and get that.
If it doesn't support Leopard (basically, G4 with processor less than 887MHz and less than 521MB RAM if I remember correctly), then buy Tiger from the Apple store, if you haven't already.
Once you've got it, with your Mac running, put the disc in and it'll get you to restart. Then, it'll upgrade the drive for you, hopefully leaving all your personal documents etc. intact.
Make sure you make a backup of at least your personal files (ideally the entire drive) before trying to do this!!