Erase my Mini not working According to the instructions?
I restarted my Mini and the instructions said to hit Command and R at the same time when the gray screen appears. My Mini started making this beeping noise and nothing happened? I then logged in and followed the instructions to go my startup disk to erase it. I selected the Macintosh HD option on the left of my window after hitting the erase tab. The format and name were then populated into their respective fields but grayed out. The only option is to "erase free space" as the 'security options" and "Erase" are also grayed out. How do I proceed to make sure my mac is erased and I can reinstall to it's original settings and sell it?
Mac mini, OS X Yosemite (10.10.3)