A proper diet is number one thing, that does not mean stop eating! Eat lots of veggies and lean meats, some fruits, few carbs and sugars. Any high intensity workout is good those include P90x, insanity, crossfit and I am sure many others. I wouldnt worry about doing tons of ab exercises, it will actually help a lot more to do strength building workouts. The more muscle mass you build the more calories your body will burn even when you are sleeping. running is good for burning calories but you will lose lots of muscle too. Also, make sure you get plenty of sleep at night.
you first need to loose weight to loose the gut. if you had a six pack already, all you need to do is loose weight. if your six pack is not there any more after you loose weight then you'll have to do alot of crunches.