That's ok, everyone has to start from the beginning at some stage.
You should see in one of your menu's something saying Terminal or Console.
If you start that application you get a command prompt which allows you to run some low level commands (don't worry)
At the prompt type:
Note the space between df
this is important. df is a command that shows what filesystems you are currently using and what space is used for each. The -k tells the df command to display the values in kilobytes.
You may want to take some time to read the following: http://rute.2038bug.com/index.html.gz
There is a lot of information there, so don't try to read it all in one sitting