Hi Winan,
You might check your /var/log tree. This is where many of your log files live. Log files can grow rapidly and consume alot os space. If you delete any logs, Linux will automatically create them again to start a new one.
Also, you take a look at the 'find' command. The find command has alot of options and will allow you to search your linux file system of file that exceed a certain limit. This is a good way to find the really big files in your file system.
Do you limit the amount of space available in your users home directories?
The following command would find files larger then (512 x 659) characters. The units are 512-byte blocks.
find / -size +659b -print