I am not responsible for anything you do wrong.
:]
1. open disk utility and locate your main hd.

2. under the partition section press the plus button and then resize the new partition you made then click apply.
3. download and install rEFIt on your mac
http://refit.sourceforge.net/4. Burn ubuntu onto a cd or buy an ubuntu cd.
http://www.ubuntu.com/5. Insert the ubuntu cd into your mac then turn your mac off and when you turn it back on hold down your "c" key (or the alt option key).
6. Now you should be in linux, click on the icon button just do the normal things until you get to the partition part.
7. So in the partition part of your install select manual, then select your linux parition ( probably sda3 if you've installed rEFIt) click edit, format it as ext3 and make the mount point /
. ignore the warning about swapspace.
8. keep on clicking next until you get to the last menu where you click advanced and click on the textfield and replace it with /dev/your patition name (probably sda3), install.
9. Then Restart your computer.
10. The refit menu should appear and you should be able to choose between mac and linux.
If u need help then u can email me or comment on this blog
Email- pwnagetutorials@gmail.com