To resize/shrink a partition or volume in Windows, open the Disk Management tool.
This can be found through a simple search in the Start Menu search bar.
Once you open the tool, either select an existing volume/partition to shrink/resize, or select unallocated space to create a new volume/partition with.
If you choose to make a new partition, you will be asked for the desired size in MB, that of which cannot exceed the amount of unallocated space on your HDD(s), depending upon your hardware configuration (and whether or not you have any separate storage locations, like an actual secondary or 'jaz' drive).
If you choose to shrink an existing partition/volume, you will be asked for how many MB you wish for the requested partition to be reduced by. The system will perform a query when you choose to do the shrink, indicating the amount of space you can shrink from your chosen volume/partition. Your MB size cannot exceed the maximum delineated, for the sake of the existing file system (unless you wish to risk overwriting files and corrupting existing the file system). This will leave you with more unallocated space when complete, which you can use to create a new partition.
Once performed, you can then create a new partition and delineate it as a sort of virtual drive, which can be formatted, allowing for read-write access. This partition is the one you can mount a new OS to, if you so choose. Just be sure to choose the correct file system format type first.
As an added note, Linux and some UNIX OSs tend to need at least 6-10 GB of free space. Mac OS X needs 10-20 GB. A new Windows or DOS-based system may need at least 10-20 GB or more. ChromeOS and Android are a bit more forgiving, needing only 8-16 GB.