Ubuntu disk image

to be on safe side you better off to ordering the disk from the official site and installing it from boot up. That way you can use ubuntu partition and have mac os x 10.5 and linux unless you want to have others?

Burn the .iso file to a CD or DVD depending upon which you downloaded. You'll want to use a utility like Toast for this preferably. You should then have a bootable disc. The LiveCD will boot up on the Mac without having to first boot into Windows.

Why do you want to run Ubuntu? Are you not aware that OS X is Unix? I can save you some time. A week with Ubuntu should convince you that OS X is much better.

To clarify, are you attempting to dual boot or triple boot your system? Because, for the latter, I don't think you can use Bootcamp. I tried triple booting before and failed, so in the end I just did away with Windows 😀

If you are trying to dual boot with Mac and Ubuntu, https://help.ubuntu.com/community/MacBook should be a pretty good guide. Although it seems complicated, all you have to do is follow the instructions, even if you don't know what the terminal commands mean. Note that you have to use a third-party boot loader rEFIt to boot up from ubuntu.