First write a list of the common foods you normally eat. Purchase a calorie counter book or look online ( calorie counter.com) . At the end of every day total the calories, carbs, fats consumed etc. Keep a runing tab for the week.Multiply you body weight by 15. This sum is the calorie intake your body needs to maintain its weight.  Anything more will result in weight gain and anything less will result in weight loss. Its really that simple. Of course there are other factors involved, but the primary factor is calorie intake.