Setup ESP8266 for Arduino

• Start Arduino IDE and open the Preferences window.
• Enter https://arduino.esp8266.com/stable/package_esp8266com_index.json (opens in a new tab) into the Additional Board Manager URLs field.
• Open Boards Manager from Tools > Board menu and find esp8266 platform.
• Select the version you need from a drop-down box.
• Click the install button.

After this choose the right board in your board manager.

• From Tools > Board menu select the right board or install it.

To read more about it click here (opens in a new tab).