The 3rd-party utilities "OmniDiskSweeper" or "Disk Inventory X" or "GrandPerspective" or "JDiskReport" will help show you where the disk space is going.
Alternatively, open a Terminal window (Terminal is under /Applications/Utilities) and enter the following command:
sudo du -x -h -d 1 /
Look to see which sub-folder is taking up the most space and repeat with that folder. If you have trouble interpreting the results, just copy & paste them back here and we can help.
(This method will work in cases where the above 3rd-party utilities fail to show where the disk space is going - i.e. the case where the files are in folders that you don't have permission to read. Note that the 'du' command may take several minutes to finish, so be sure to wait until you get the prompt back.)