Hello Carlo! It's rather easy to do. Once I needed install second OS. And I was recommended to use Disk Director
. This partition manager software allows me shrink my first OS partition "C" (it was 20GB) to 10 GB. Then I create new partition "D". But check the letter of partitions and rename, if it need. In partition manager make partition "D" primary and active, and make sure that this partition has correct file system (NTFS or FAT32) it depends of what kind of OS you'll install on it. Don't forget to activate OS selector, reboot PC and now you can start installing new OS. Good luck!