This is an introduction with all the problems with these gas sensors: http://arduino.cc/playground/Main/MQGasSensors
So you have to do the burn-in time. After that you have to find a good value for the resistor. After that you need a standarized mixture to measure at what analog value the alcohol percentage is. The code is just analogRead() to get the analog value.
If you have done all that, the sensor needs time to heat-up. That could be 2 minutes. This is needed before using it every time.
For analyzing breath, the MQ303A is a much better choice. It doesn't need to be on for 2 minutes before use.