Here is the required procedure should you have a Late-2011 iMac or later, assuming that is what you have based on your profile.
Internet/Network Recovery of El Capitan or Later on a Clean Disk
If possible back up your files before proceeding.
- Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
- The Utility Menu will appear in from 5-20 minutes. Be patient.
- Select Disk Utility and click on the Continue button.
- When Disk Utility loads select the target drive (out-dented entry with type and size info) from the side list.
- Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
- Set the partition scheme to GUID.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then click on the Done button when it activates.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
You need a fast and reliable Internet connection. If you have Ethernet, then use it. If all you have is Wi-Fi, then be sure it is connected. The Utility Menu screen should show a Wi-Fi menubar item from which to select and log into your Wi-Fi.