Use Carbon Copy Cloner from Bombich software (not Time Machine) if you want to dual boot.

Backup your Mac OS X install to an external FireWire or USB drive.

(I am not using the English localisation of neither Mac OS X nor Ubuntu, so the exact worthing of buttons and such might be slightly off in these instructions, but the gist of it is right.) If not, then it will just need to be large enough to fit the live. If you want to dual boot, you will have to make it as large as you want your later Mac OS X partition. Before partitioning, you will need to know if you want to dual boot or not to decide how large the partition you will use for the Live DVD should be. If you want to dual boot, you can restore your Mac OS X install to the partition we used for the Live DVD after installing Ubuntu on the other partition, but if you just want Ubuntu, then keep the other partion as a restore partition. What you basically do is install the Live DVD to one partition and install onto the rest of the disk from that partition. (PowerPC Macs used APT, not GUID partition tables, so only after the Intel transition did the operating system get the ability to see GUID disks.) The other Mac must run Mac OS X 10.4.7 or later (PowerPC or Intel doesn't matter), since otherwise it will not be able to 'see' the disk in the MacBook.

Maybe borrow another Mac from somebody if you don't have two. (You also need this backup of Mac OS X to extract the firmware for your iSight webcam after the install.) And you will need a disk to backup your Mac OS X install.

If the other Mac has FireWire 800, you will need a FireWire 400 to 800 cable. You will need another Mac with FireWire and a FireWire 400 cable. This is a method to install Ubuntu on your MacBook using FireWire Disk mode. I have made it work on an iBook Clamshell after running late versions of Tiger (but not after running earlier versions), but I have never been able to do it on an Intel Mac, except for a Mac OS X install disk from an USB optical drive (but not Ubuntu live dvds from the same drive). Booting from USB is hit or miss with Macs (both PowerPC and Intel). I had the same problem with the optical drive in my MacBook from late 2006 after pouring milk into it late one night and did get a working install this way.

How to install linux on old macbook