Looking to installin Ubuntu on MacBook 4,1

[bbqamazing]

Hi, I'm new here. Never been a Linux user (except once I bought a used iPod that had Linux installed). I have a MacBook from early 2008 (4,1) with a Intel Core 2 Duo processor and I'm running OS X 10.5.8. I have 120GB of internal storage. I also have a 1TB external hard drive. I'm wanting to run a dual boot of OS X and Ubuntu. I've backed up my computer onto the hard drive, I've downloaded Ubuntu Desktop 12.04 LTS, bought some blank CD's, and now I'm a little stuck. I guess I need to partition my hard drive? I don't have Boot Camp, and I guess I can't get it, because I'm running 10.5. I'm a little wary of doing it in Disk Utility, because I'm not sure I understand what I'm doing. I only have 21 GB of free space on the laptop. Does that mean that I can only partition 21 GB of space for Ubuntu? Can I boot one OS from the external? I think I would prefer that.