files - if you mean documents, windows, music, videos, you will need to copy them onto a USB or other hdd in PC
Programs - you will need to reinstall them all on a clean install
Games - if you have steam/origin libraries, you can copy them to another hdd and copy them back on again on new install
wow - can copy folder and reuse it later
everything on C drive - depending on age of PC, win 10 may decide it wants the hdd C is on to be formatted as GPT. What that means is you have to delete all the partitions on C and let windows 10 recreate them. So remove anything you want to keep off the hdd C is on before you do this.
download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
change boot order in BIOS so USB is first, hdd second
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and reactivate once finished
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here)