I am trying to erase an older MacBook Pro with Intel Core Duo and OS 10.4. I've tried every combination of key presses during start-up without ever getting to Recovery mode. I don't know what to do next to get it erased. Help??!

I believe that Mac OS X version 10.4 Tiger is too old to have the advantage of the Recovery boot options. I think that became available with Mac OS X 10.7 Lion.

Ideally, you should use the grey label Mac OS X install CD/DVD discs that the machine shipped with. You would boot from that and then erase the internal hdd and reinstall the Mac OS X version that the machine shipped with.

If you no longer have the original install CD/DVDs I think the best you can do is use System Preferences > Users & Groups to create a new user account with admin privileges. Then log into that new, empty account and use, again, Sys Prefs > Users & Groups to delete all of the other user accounts.

Finally, use Disk Utility to Secure Erase any free/empty space on the internal hdd. Use a multiple-pass option that will write zeros to the empty space.

I think this is right, but it's been a long time since I played with OS X versions of that era.