Hi Howard
Here is a simple approach to regaining space on your Hard drive.
Your Hard disk contains
1. The Mac OS X Operating System, and
2. your stuff, which may be documents you have created in Applications, photos you've taken, music and videos you have downloaded, emails you have received.
It is these that are first things to look at when looking to recover space as they are most likely the cause of the overfull hard drive.
Of these, copy those 'documents' to a separate folder and move them onto an external disk or burn them to a CD. Check the moved documents still open, and if so, then trash the originals on your hard disk and empty the trash.
Eg. Open the relevant application, say iPhoto. Export so many photos to a new folder. In that new folder check that those photos are there and they open. Burn them to a CD. Open the CD and check that they are there and they open. Satisfied return to iPhoto. Trash those copied photos in the iPhoto Application. Empty the trash in iPhoto.
For a thorough examination on OS X maintenance have a look at
Macintosh OS X Routine Maintenance by Randy B. Singer
There is a lot in there, so perhaps copy it and work through it at your leisure.
regards roam