I have an old iMac that I would like to format and install Ubuntu over. Installation over Microsoft Windows is automatic, but not so with the iMac. Any help would be appreciated. Thank you.
Turn the Mac on, put the Ubuntu CD/DVD inside the SuperDrive (or whatever type of optical drive your Mac has) and immediately press and hold the option (alt) key [this should be done before the gray background appears]
After a bit your Mac should show up all boot options including the internal Hard Drive and the Ubuntu CD/DVD.
From there on everything is straight forward: Boot Ubuntu, format the Hard Drive, install Unbuntu, etc...