If you want to do that on the main partition that you are running, you can't. To partition it, it cannot be mounted. So the best bet is to boot into a Live disk (doesn't matter what distribution) and then partition it. This time, partition it and make the OS size really small and put everything else into volumes. That way, if you need to do more partitioning or disk management, you don't have to shut everything off, you can just mount / unmount.