If you are trying to figure out what is using up all your disk space, you might use a tool like Disk Inventory X. However, if you run it as yourself, you won’t get accurate results for the entire system. But you can run it with root privileges. To run Disk Inventory X with root privileges you would type (and tabsudo /Applications/Disk\ Inventory\ X.app/Contents/MacOS/Disk\ Inventory\ X

Use sudo with caution