How do I erase the internal Hard Drive on my Mac loaded with OS X Yosemite?

My family has been using this 09 Macbook Pro for 6 years now and my dad just handed it down for me to have. It is loaded with OS X Yosemite which shouldn't make any difference in the process of erasing the Hard Drive Macintosh HD, but as I go into Disk Utility and select the drive, it fades out all of the options and I'm not sure why. Does anyone know why this is happening and how to fix it? Or am I just missing something.