If you're using Gnome, go to Applications -> System Tools -> Disk Usage Analyzer. This utility can scan you entire filesystem and show you the folders that take the most space.
Alternatively, you can use this command :
This will show you all subfoders, ending with the ones taking the most space. So basically, to scan your entire filesystem, you should run it from "/". To scan your home folder, you should run it from "/home/you", etc.