In short order here is a list of instructions to follow:
1. Determine if your Mac has an Intel or PPC (Power PC) CPU.
2. Based on the answer to #1, go to the Ubuntu site and download the appropriate ISO file for Fiesty Lawn, the current iteration of Ubuntu.
3. Follow the instructions to burn an ISO CD from the file you downloaded. This is very important as you cannot load an operating system from a "regular file copy" on a CD.
4. Place the ISO Ubuntu CD in the Mac and turn it on.
5. While it is booting, hold down the "C" key until you see the Apple logo then release it. After a few seconds or minutes a dialog box will come up giving you the choice of which file to use to boot the Mac.
6. At this point your only choice, unless there is an ISO file on the hard drive, will be the Ubuntu CD. Click on it and wait for the Mac to reboot.
7. Once it reboots, follow the instructions to load Ubuntu.
Ubuntu is a wonderful Linux distro and it looks very cool on a Mac. Good Luck.
DrEarp