the junk food isn't so much of a problem, after all a calorie is a calorie. The problem is a combination of overeating (seriously do you need 3 straight meals plus snacks every day??), and inactive lifestyles. One actually has to make time to exercise as sitting at a desk doesn't burn off those meals you've been eating, whereas in past times people would be burning off a shed load of calories as part of their work.

Work out your basal metabolic rate, add in your energy use from day to day activities then eat a diet that will give you an appropiate number of calories...and stick to it.

Energy in will always be stored unless it's the same or less than energy out.