I wish I knew Mac's better.
The startup disk on a MAC is the main hard drive. It seems as though your hard drive is near capacity.
Start with the easy stuff...
1. Empty the trash. If you delete a lot of stuff and never empty the trash, it's not really deleted.
2. Move pics, vids and audio stuff to an external drive. (Or add an internal drive)
3. If you use an email program, make sure that you are not deleting emails and leaving them in the trash in the email program.
4. Not sure if this applies to Macs but, you should clean out your temp files too.
5. Clear your browser cache too...
A little more complicated, but I believe Macs have a disk utility that you can use to "Erase Free Space". You might poke around and try that too.
That's about all I can tell you right now. If I had a Mac in my lap instead of this Compaq I could do more... sorry.
