Once you have your openhab installed, you will need to use the MQTT Binding addon in order to use MQTT with openhab.

In your openhab configuration file (probably "openhab_default.cfg") you have to include the configuration of the MQTT browser. I've included:

mqtt:mosquitto.url=tcp://localhost:1883
mqtt:mosquitto.clientId=openhab
mqtt:mosquitto.qos=1
mqtt:mosquitto.retain=true
mqtt:mosquitto.async=false

I'm using message retain and qos=1 so everytime I read the MQTT from the ESP8266 there's always an ON or OFF message (that the esp8266 obeys).

In the items folder, in the config file, you can add something like this:

/* MQTT Sensors */
Number Temperature_GF_Living "Temperature [%.1f °C]" <temperature> (Temperature, GF_Living) {mqtt="<[mosquitto:/sensor1/tem:state:default]";}
Number Humidity_GF_Living "Humidity [%.1f %%]" <water> (Humidity, GF_Living) {mqtt="<[mosquitto:/sensor1/hum:state:default]";}
Switch Light_GF_Living "Led" (GF_Living, Lights) {mqtt=">[mosquitto:/sensor1/led:command:on:on],>[mosquitto:/sensor1/led:command:off:off]";}

Restart openhab and everything should work.