If I get to the screen to delete my disk file how do I erase it?

[iMore Question]

I'm trying to reset my Mac, I have got to the screen where you choose the disk file and I would to erase it but the erase button isn't allowing me to erase it

 

[bamf-hacker]

You can't erase the active partition if you are logged into the Mac.

Reboot and go to the recovery options by holding Command and R when the Mac is booting up. From there you can get to disk utility and work on the hard disk.