How do I free up space on Windows 10 when c drive is full?

I have an 12-year old Windows 10 PC with a small size SSD drive. The C drive is 80GB and now only 7 GB free space is left. This is a very common issue as the SSD volume is quite small at that time. Now, the computer starts to run very slow. I doubt it is because the c drive is full and not enough free space for running programs and software. Is there any effective way to let me easily and safely free up on Windows 10? When I was trying to get a reliable answer from search engine, there are som many junks out there. I already wasted a couple of hours on this.