Start with the Windows Disk Cleaner. You can skip a step by holding the Windows key and pressing R. That opens the Run box. Type: cleanmgr and then while holding down the Ctrl + Shift keys, press Enter. That gets the Disk Cleaner to run and show System files that can be removed. Your biggest savings may come from removing previous Windows versions, possibly over 20gb. There are also free utilities that can help too. Google "clean hard drive".