5. remember, this will wipe anything on your computer drive, so be sure to back everything up first.Here is the answer:
1. You need a way to boot the Mac using an EXTERNAL drive that is bootable.
2. You then open Disk Utility and "aim it" at the internal drive (topmost item in DU's list, and you MUST first choose "show all devices" from the popup menu (if there is one)
3. You then click the "erase" button and choose "Mac OS extended with journaling enabled".
4. DU will then erase the disk and "format" it.