clearing disk space in CLI?

[sylvaine]

Whilst trying to get my GUI back, I got the error message "Keyboard initialisation failed. This could be a missing or incorrect setup of xkeyboard-config.", which google tells me means I need to free up disk space.

As far as I know, I need to use the rm command to do this, but how do I figure out which files to delete without my GUI? I'm deathly afraid that I'll delete something I need.

(Please bear in mind that I'm an utter noob when it comes to Linux.)