If it is windows you are in, go to disk management, select the partition and right click on it, there should appear an option for resizing, set the final space you want in the existing partition and after that you'll get the rest available to create the new one. If you don't have enough space available to free up the amount you want, you'll need to save your files to another drive temporarily and erase it on the disk so you can free up more space, and then just copy everything back to the new created partition. Hope this helps.
Another option is to use an app for this, there are some you can use, one I use a lot is a boot cd for the app Gparted.