For extended support, choose Ubuntu 12.04 LTS
Ubuntu 12.04.4 LTS is a long-term support release. It has continuous hardware support improvements as well as guaranteed security and support updates until April 2017.
Full installation instructions
Ubuntu 13.10 will be supported for 9 months and includes cutting-edge new features that make your music, videos, documents and apps much easier to access.
Full installation instructions
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Also refer to the detailed guide on How to burn a DVD on OS X
Also refer to the detailed guide on How to create a bootable USB stick on OS X
Try Ubuntu before you install it
If you’re not yet sure about installing Ubuntu, you can try it out without affecting your current system.
You can try Ubuntu without actually installing it on your computer’s hard drive. You can do this by starting up your computer with either an Ubuntu DVD in the drive or a USB stick with Ubuntu on it in a USB port.