The oldest operating system you can install in it natively is the version and build of Mac OS X that shipped with it.
So if it shipped with 10.4.7, you can install that version from he install disc that was included with the Mac, and if the version included was later than 10.4.7, then whichever version was included is the earliest that will work with your system.
Insert the disc 1 in the Mac, and select to boot to the installer, or boot and hold C at startup to make it boot from the disc.