Thanks to everybody in advice for reading my post.
[LEFT]I followed the instructions found in internet, the ones of this community too.
I installed rEFIt to have the dual boot at the start-up of the machine, but I confess I did not understand very well how this software works.
I opened Disc Utility and made a 100 gb partition (my HDD is 500 gb in total). I rebooted the machine running the dvd of openSUSE.
It took some tries to make the computer to read the dvd and to install the OS. I do not remember if I clicked on “Install” or on “Boot from Hard Drive”. The fact is SUSE set-up went well and I played a little bit with my new OS. Then I turned off the machine and discovered that SUSE is not running without its DVD.
[LEFT]I thought I made something wrong. I switched back to OSX to uninstall rEFIt and to delete new partition (which, in the meanwhile was cutter down in three/four smaller pieces - some gigabyte each - and a bigger, 90 gigas of emptiness). I was trying to restart everything to fix my mistakes.
A) I didn’t understand what went wrong;
B) Disc Utility can’t make my partition to return to 500 gigabytes. It continues to show me 400 gigas of free space;
C) I read all the beginners tutorials. Probably I’m not a genius but I can’t understand clearly the game I’m playing. I always see that lots of stuff is required to understand basic guides too.
How can I fix everything?
I am really sorry for this uncomfortable request, thank to all of you.