How to boot Mac from CD from verbose command line
Mac hardware will always boot to an os chooser holding down option. You of course need a USB / thunderbolt / FireWire drive with bootable media at that point. 


https://support.apple.com/en-us/HT201255


Newer Macs use internet recovery to net boot and from there you can repartition any drives and reinstall all drivers needed to boot macOS and start again. 


Accessing Internet Recovery


For a PowerBook, you might also use cd / DVD media since internet recovery isn't an option for PPC hardware.