[Midvale]

After you've downloaded the Ubuntu iso file create either a boot dvd or usb thumb drive. You cannot merely copy the file onto a dvd or drive; that won't work. It needs to be burned or created as a Disk Image: Create a USB stick on OS X | Ubuntu

Insert the DVD or drive and restart your mac while holding down the Option key. That forces it to check other devices for startup besides the hard drive. The Ubuntu device may be displayed as Windows. Just select it and hit Return. Ubuntu should boot up and you can run it to try it out or install it.