D O A BACKUP. At least of the sensitive data on the partition, you want to shrink. Neither I, nor geparted can be held responsible if you screw up.
Now to the procedure using gparted:
The picture is a little messy but don't get scared (
you'll be moving with your partition to your auntie and uncle in Bel Air. Wait wat? XD). Just stick with this for a moment and follow the instructions below. Start looking at the image in the top right corner. Then follow the numbers in ascending order.
Here is a text version of the steps numbered as in the picture:
sudo apt-get install gparted
This will not work with all partitions. It can be more complicated for Mac partitions or logical volumes.