Get another disk, an external if you can't add another internal, and move everything not system related. Ideally, the system drive should contain only the operating system and applications. Everything else, all your assets and working files, on different drive(s).
Run the disk cleanup utility that comes with Windows/MacOS. It's usually very effective.
If that's not enough, there's always still a lot of stuff in your user account that can be moved or removed. Things tend to accumulate there. But that requires some tools and knowledge of what you're removing - not because it will harm your system, but because you might actually need it to work efficiently.
How big is your system drive? A "standard" configuration of operating system and applications (including a range of CC apps) should normally not take up much more than 100GB or so.