I don’t have a mac, though I do use Ubuntu (Lucid Lynx) on computers that once ran Windows. I much prefer using Open source, of which Ubuntu is but one distro (distribution). There is a link to downloading Ubuntu on the link above.
Basically, download Ubuntu to your hard-drive. Burn the file as an ISO (image file). Set computer (I assume the same for Mac) to use CDROM drive as first boot, and restart computer with the disk in the drive. You can then use the disk as a "Live Disk" working entirely in RAM so nothing is altered on the Mac’s hard-drive – or use "Install".
Either accept sharing the partitions between Mac and Ubuntu (dual-boot) or let Ubuntu use the full hard-drive and so make a single partition.
There is a programme (on the ISO disk) called WUBI that (on a windows computer, at least) allows Ubuntu to be installed as a file, so nothing is altered – and a dual-boot system is set up. To get rid of Ubuntu, in this case, simply use the Add and Remove Software utility of your computer.
http://wubi-installer.org/faq.php
The truth of the matter is, installation instructions vary slightly (but only very slightly) for different distributions of Linux, and furthermore, more and more people are announcing the obvious point that an operating system is not a rational decision but a very subjective one. Most people use Ubuntu. And many people either feel it is the only Linux Distribution or that it is not fit to be called a Linux Distribution (I’m definitely outside both of those camps. While I dislike its update policies immensely, I strongly respect that most of the Ubuntu-specific development going on is directly related to Security and other Newby issues — a lot of its interface is inherited from Debian and other distros (subject to different licensing policies which have resulted in, among other things Firefox being called Iceweasel on Debian) and this commonality, aside from being reassuring, points up just how well this interface has evolved).
Anyhow, I have two tutorials on Dual-Boots, one Ubuntu-specific, one more generic and dealing with MEPIS, another Debian Derived OS, written by one of the best technical journalists who covers linux out there. Please make sure you have decided on which Linux Distro you want to install, and look up SPECIFIC information for installing that distro. The two bootloaders, LILO and GRUB, both give you the options of booting into Windows. Grub is easier to run, even for those of us who cut our teeth on Lilo, and is recommended.
I have heard of a Windows Ubuntu-installer called Wubi, but of course I have never used it (Linux Ubuntu Geek has been recommending it here). Get more information at:
http://wubi-installer.org/
Now for LiveCDs. The advantage is once you’ve set your bios to boot direct from CD (which you have to do for installation disks anyhow) you get essentially a full fledged linux system running in memory. It usually takes at least 300M of memory and runs more slowly than it would if you were running it from the hard drive, but there are NO Installation hassles, and it gives you the chance to learn about Linux before making the commitment to install it.
The one which made LiveCDs popular and which is really awesome is Knoppix. You can probably buy a book about it in your brick-and-morter bookstore, with a copy of the CD in back. While it was created by a computer consultant for his own use while on house calls, it features a full-featured Debian desktop with Iceweasel (firefox), a word processor and even card games. You can get more information at:
http://www.knopper.net/knoppix/index-en.html
Slax is a basic or generic distro which is not too complicated for a newbie and is actually very extensible. I tend to prefer 1.5.9 over more current versions but you can find it at:
http://www.slax.org/
Powered by Yahoo! Answers