OPTION #1:
You need a genuine Installer Flash Drive that contains a compatible
OS for your Macbook.
Follow the procedure below to prepare the Installer....
https://support.apple.com/kb/HT201372
How to create a bootable installer for macOS
Insert the Drive and power on while holding the OPTION Key.
You should see the the OS Installer volume icon.
Click on it and follow the installation instructions.
OPTION #2:
https://support.apple.com/en-us/HT204904
How to reinstall macOS from macOS Recovery
Connect a reliable Internet feed to your Laptop,
Or... Make available, a strong Wireless Internet feed.
Power ON while holding the COMMAND + R Keys.
A Globe Icon will appear. Then a Wireless Connect option.
SELECT your Wireless Internet access and enter the PASSWORD.
Your Laptop will then take about to 15 minutes to download
the Recovery Utility.
SELECT install OSX.
Follow install instructions.