Directories and its size

which directories and trees take up all the diskspace?
du -sm $(find /start/dir/* -type d -maxdepth 1 -xdev) | sort -g

If you want more human readable output try:
du -ha /var | sort -n -r | head -n 10

you want to see ALL directories in the tree
find $1 -type d | xargs du -sm | sort -g

To show all directories size including sub directories, type

du -h

To calculate the current directory size you are in (-s stand for summary)

du -sh

To show all the 1 level sub directories size (which you are not interested at sub sub directories.)

du -sh *

To show the size of specific directory

du -sh /home

To show the size of all sub directories of a specific directory

du -sh /home/*