I had 250 GB free on my hard drive a week ago, I have done almost nothing and now it's 80GB? I have only browsed the internet, and added a few files around 2GB. When I look in / (with hidden files) and take all the sizes together it just becomes about 730, but I have a 960GB hard drive with only 80GB free.

Why is this and how can I find out?
If you homebrew, do the following:

brew update
brew install ncdu
cd /
sudo ncdu


ncdu is an ncurses-based command line tool that makes it really easy to see what's taking up space on your drives.  It allows you to drill down from within the Terminal itself and delete stuff if you need to.

If you prefer the GUI method, the best application for this is DaisyDisk.  It is a paid application that offers a free trial.  It's available in the Mac AppStore and also from the link I posted to the official website.

There is also a free alternative, which is almost just as good called Disk Inventory X, which is available in the HackStore and from the official link I posted to its official website.

Both DaisyDisk and Disk Inventory X are like WinDirStat or TreeSize, if you have ever used either of those on Windows.