You can use two commands. The first is to use
df - which according to the Ubuntu Manpage:
df - report file system disk space usage
Usage works like such:
Which should output something like this:
Filesystem Size Used Avail Use% Mounted on /dev/vzfs 20G 3.5G 16G 18% /
-h flag provides human readable output (which makes reading of the output - easier).
A second tool is with
du which is a slower approach but will give you a better break down of a per directory. Information on that can be found in How to determine where the biggest files/directories on my system are stored?