The startup disk is your boot disk or boot volume. Usually named, "Macintosh HD."
Freeing Up Space on The Hard Drive
1. See Lion's Storage Display.
2. You can remove data from your Home folder except for the /Home/Library/ folder.
3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
4. Also see Freeing space on your Mac OS X startup disk.
5. See Where did my Disk Space go?.
6. See The Storage Display.
You must Empty the Trash in order to recover the space they occupied on the hard drive.
You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.