I have a Mac PowerBook G4.
It has the Mac OS X Version 10.5.8 on it.
I would love to change it to Ubuntu but the last time I tried it wouldn't register the disc.
Any ideas if I need an older version or what needs to be installed in a certain way?
I'm not sure what you mean by "wouldn't register the disc" but to boot an older iBook / PowerBook from the optical drive, just hold down the C button as soon as you power on.
Another option is to boot into OpenFirmware. This can be done by holding down Cmd-Option-O-F as you power on. Once OpenFirmware has started (a white screen with a short message and a simpole prompt) enter the command: boot cd
I hope this helps. I'm going from memory.
Edit for Addition:
Also, just a warning: Years ago when I used Linux on a MacBook there was no Linux driver available for the Broadcom wifi chipset that Apple used in their laptops. At the time, the only option was to use a usb attached wifi device. This may no longer be an issue.
A start would be this tutorial for Lubuntu 12.04 http://powerpcliberation.blogspot.de/2012/10/lubuntu-install-guide.html.
Lubuntu is optimized for old computers.
the latest Images you find here: http://cdimage.ubuntu.com/lubuntu/releases/ (currently 13.04) use the "Mac (PowerPC) and IBM-PPC (POWER5) desktop image" in http://cdimage.ubuntu.com/lubuntu/releases/13.04/release/
I tried it on a PowerMac G4 and it seems to work. (although, while it's trying to install, I get the message
But that seems to be due to an error on the old harddrive I inserted into the G4, I see some I/O errors in syslog when I change to the console with Ctrl+Alt+F1 )