The usual way to have a look at your disk usage is with the commands df and du. df shows a summary of how full your disk is (use df -h to get more human friendly figures, in GB rather than KB). du shows the space taken up by each directory and subdirectory (starting from the current working directory). du -sh is more useful: -s summarizes the usage of the current working directory, and -h uses human friendly figures.
However, using du to work out where the big lumps of disk usage are can be time-consuming; you have to start off with du -sh /* and gradually drill down through the largest directories. A really useful graphical alternative is Baobab, which is installed by default in most Gnome-based Linux distros (including Debian and Ubuntu). It's also available for Mac via DarwinPorts.
Boabab will scan whichever filesystem or folder you point it at (or even a remote folder on another machine), and provide a graphical representation of where the disk space is being used. When I ran it on my rapidly filling up netbook I discovered that /var/cache/apt/archive was the main culprit (handled immediately with a simple sudo apt-get clean!).
With modern disks, you might think that disk space is unlikely to be a serious problem, but in my experience data always seems to want to expand to fill the space available. Baobab is an excellent tool to help you quickly work out where you can do some spring-cleaning. If you don't use Gnome, a similar alternative is KDirStat, which goes slightly further than Baobab in also including facilities to help you clean up your disk.
Juliet Kemp has been messing around with Linux systems, for financial reward and otherwise, for about a decade. She is also the author of "Linux System Administration Recipes: A Problem-Solution Approach" (Apress, 2009).