Trying to erase files from my HD on Mac Mini 10.3.9. It's not working. Help?

Hey all. I'm on a PowerPC G4 Mac Mini, running 10.3.9.

I am trying to delete files permanently from my hard drive, so I go into Applications > Utilities >Disk Utility. I've selected Mac HD, but the option to erase is greyed out and unclickable. How do I fix this, so that I can delete my sensitive information?

Any help would be appreciated. Thanks.

In erasing the files, are you trying to get rid of 1) everything, or 2) just selected material?

If 1), then you'll need to boot from and use the Panther Install Disk 1. Thereafter Erase the Mac HD completely. Then reinstall Panther.
If 2), then using Disk Utility isn't the way to do it. You need to drag/send the material to the Trash and then Empty. Use Disk Utility thereafter to Erase Free Space, but you'll need 10% of your hard drive capacity free for that to work properly.