1. Boot to Internet Recovery Mode (<kbd>shift</kbd><kbd>alt</kbd><kbd>cmd</kbd><kbd>R</kbd>)
2. Open Disk Utility and erase your internal disk
3. Install macOS ([the macOS that came with your Mac, or the version closest to it that is still available will be installed][1]). 
4. After reboot and configuring your system download and install macOS Sierra.


  [1]: https://support.apple.com/en-us/HT204904