Use Disk Cleanup on the General tab of of the Properties dialogue box for the C: drive. Do use the Cleanup System Files option. Enable compression. Uninstall programs you do not use.

Disable the ability to hibernate. That will remove a very large file called hiberfil.sys. (Note: This will also disable Windows 10’s Fast Startup feature. I’ve always preferred putting the machine to sleep anyway.)

Install WinDirStat to help you see any large blocks of memory which are being used unnecessarily.

Here is piece of advice you are not likely to find anywhere else: Restore points can eat a lot of space, and they are not dealt with completely in Disk Cleanup. On the second tab after you go for System Files, there is an option to delete all but the most recent; but that is not much help if it turns out to be a biggy. (I had one that was 5GB once.) To remove ALL restore points, go Control Panel -> System -> System Protection -> Configure. At the bottom, there is an option to delete them.

There is another folder from which you can delete stuff: C:\Users\[user name]\AppData\Local\Temp. I have often removed the contents of that folder without causing any problems. (I normally copy it to an external drive just in case, but I never found that I had to restore anything.) Even though Disk Cleanup offers to remove what it calls “Temporary Files”, the ones in this folder are not included. Over time, the folder can get pretty big. If there are a few files which File Explorer balks at deleting when you try it, just leave them be. (A few could be currently in use.)

In your web browser, delete history, cache, and cookies.