There are plenty of tools with which you can check your disk space.
However, Linux already has a built in function to show you just what you need to know.
Open a terminal window or push (ctrl+alt+F1 to go to console) and type:
You will see something like this:
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/hdb2 12262868 2383044 9256904 21% /
/dev/hdb1 101086 11096 84771 12% /boot
none 95580 0 95580 0% /dev/shm
This one is bit unreadable, because size is represented in 1K-blocks. Lets try to make it clean and more readable:
Filesystem Size Used Avail Use% Mounted on
/dev/hdb2 12G 2.3G 8.9G 21% /
/dev/hdb1 99M 11M 83M 12% /boot
none 94M 0 94M 0% /dev/shm
Now the size is represented by megabytes and gigabytes.