You normally use 2 partitions at least - better 3: One for the system, one for swap, and the third for home, on a single-user desktop system.
If you have plenty of RAM, you don't need swap, except for hibernation, which is a nice thing to have. You normally choose 1.5x your physical RAM-size for SWAP, if you don't have reason to do more or less.
So if you didn't use your second partition so far, I would use the opportunity to create a swap space, by splitting the free partition.
A separate partition for home is useful if an major upgrade fails. You can install a new system without touching your home, keeping all data, all settings, all emails and so on in place.
But you have to investigate, why you can only use 15% of the current partition.
For creating the new partition, I would suggest
to repartition the free partition.
Later you format the one partition, which shall be the new /home with
for an ext4-filesystem. You copy your home-folder content over, preserving attributes and permissions - the easiest way is to use the midnight commander - to preserve date/time as well, so that your files don't get the time/date of copying.
Then you mount testwise the new partition as /home. This will hide your old /home which makes it a bit tricky to delete it later, but you don't want to delete it, before it works.
For deleting the old content, you unmount /home, then you rename /home to /home-old, and create an empty /home folder for remounting the new partition. Then you can safely delete /home-old.