I need to get my computer back up and running like new - what are some good ways to clean up an SSD of unwanted junk?

So the title says it all.

I got myself a good old Windows 8.1 desktop with a recently ruined HDD, a new 2tb HDD and the SSD (120gb) that I use as the main drive for the Windows installation. However it's full. Always is. I can clean it up by 2gb and then by the end of the day it's back at the old 200mb storage left. I know what about... 40-50gb of the 120gb, give or take. That, however leaves up to 70gb (and more) that I have no idea what is/ can't access. (Maybe it's Onedrive using up space eventhough I told it to keep everything on online-mode only??).

How do I go about cleaning up this mess?