Well, actually you should start by identifying the big folders.
will tell you the disks mount point and occupation.
du -sk /any/directory/* | sort -n
will show you the folders size. 'cd'-ing in the biggest ones, you can have the sort for subfolders. It's up to you to know in your data what is deletable or not.
Also, have a look in the temp folders, basically ~/tmp for your home temp directoy, and /tmp for the system temp dir.
Some log files may also get big (/var/log), but maybe logrotate is called periodically on your system to manage log files.
grep -R logrot /etc/cron*
shall tell you if it is called by cron or not.
Browser cache may also get big, fix the max allowed size in your browser preference dialog.