To install the ESP8266 board in your Arduino IDE, follow these next instructions:
1) Open the preferences window from the Arduino IDE. Go to File > Preferences
2) Enter http://arduino.esp8266.com/stable/package_esp8266com_index.json into the “Additional Board Manager URLs” field as shown in the figure below. Then, click the “OK” button.
3) Open boards manager. Go to Tools > Board > Boards Manager…
4) Scroll down, select the ESP8266 board menu and install “esp8266”
5) Choose your ESP8266 board from Tools > Board > Generic ESP8266 Module
6) Finally, re-open your Arduino IDE