Install rEFIt (a bootloader - basically a nice little GUI for selecting which you want to boot when you turn your mac on) then just partition your harddrive (one partition per OS, but Linux needs 2, a smaller swap partition - 1GB will do. And don't worry, you can leave your osx partition the way it is now, nothing will be lost) then just insert the install DVD for the OS, reboot your mac, hold "c" as it turns on (to boot from the DVD) then just go through the onscreen instructions for installing the OS (just do the advanced options though, so you can select the right partition for it to install to). As long as you never choose your osx partition to be the install location, you wont lose any files etc

And then you're done :P There are some good videos out there that will explain it a lot better than I did (they make it pretty easy, even the technology impaired duck could do it)