How can I erase my Macintosh HD?
Go to your Disk Utility [it's in Applications > Utilities > Disk Utility] or by Spotlight Search. 

Click on the Volume where your Mac HD is on. 

Click on Partition tab and select the corresponding partition you want to erase. Now click on the minus (-) button in the bottom bar. 

Note this option will be grayed out if you don't have more than 1 partition. Instead you want to erase the drive. Click on the Erase tab of Macintosh HD and erase accordingly.