Only do that in your Home folder which means only you know what is safe to delete. Use the Finder to navigate through the folders in your Home directory. However, stay out of the /Home/Library folder.
Freeing Up Space on The Hard Drive
1. See Lion's Storage Display.
2. You can remove data from your Home folder except from 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.