It's really simple. You use the Bootcamp Manager that you find in the Utilities folder, and set it up to split your boot volume into two partitions. When it's done with that, it'll tell you to either insert your Windows disk or to quit the Manager and install Windows later. Do the latter, and download Ubuntu 9.10 from its website (www.ubuntu.com
). At any given point, you can insert the Ubuntu CD in your drive and press C at startup (during the grey screen), and it will install Ubuntu. Make sure to set up the partitions manually in the Ubuntu installer and place the installation in the partition you set up in the Bootcamp Manager.
I can recommend Ubuntu, and the installation procedure is simple if you've done this kind of thing before, but you have to be sure you know what you're doing. Maybe get a tech savvy friend to help you? It'll be a cakewalk for anyone who knows a little bit about partitioning and stuff.