du to determine the largest directories, you can start from /, using a simple filter helps much, for example:
du -sh /usr/local/* | sed -n /[0-9]G/p
du -d1 -h. The d1 drills down 1 level of folders. If you change that to d2 it will drill down two levels. The more you drill down the longer it can take for the results to so. The -d1 is a great way to get a high level view of the various folders.
cd / && du -smx * | sort -n