I definitely agree with what Eli said. There are cleansing diets you can approach that will keep you eating food all day long, but will still start to detox your body for you. The Acne was probably a reaction of stress and vitamin deficiencies in your body. Also, if you've had a unhealthy past, it would be a good idea to get a blood panel done by a docotor, and undergo at least your first cleanse or fast under medical supervision.
I'd just like to point out one more thing...if you choose to fast, will this be about depriving your body of something, losing weight, or detoxing? You really need to consider that before you go ahead with it.