There are two flags that you will want to use with this command:
The -h flag will specify that you want the sizes to be listed in human readable format like K for Kilobyte or G for Gigabyte.
The -s flag will produce a summary instead of giving you information about every subdirectory.
You give the directory you would like to check as the argument. If no directory is specified, it will you your current directory.
The syntax for this command is:
du -sh directoryname