$ df -k
to see the capacity of each disk mounted on your system, the amount available, and the percentage of space already in use.
File systems at or above 90 percent of capacity should be cleared of unnecessary files. You can do this either by moving them to a disk or tape that is less full, using cp to copy them and rm to remove them, or you can simply remove them outright. Of course, you should only perform these kinds of "housekeeping" chores on files that you own.