If you eat too many calories you gain weight. If you eat fewer calories than you need you lose weight.

If you eat 3600 extra calories that you need, you will gain one pound of weight/fat. If you take away 3600 fewer calories than your body needs, you will lose one pound of weight/fat. Now you know how many calories/how hard you have to work to gain or lose weight/fat.

When thinking of calories per day you must realize that if you burn 500 calories per day you will lose 1 pound per week. If you do that AND cut out 500 calories from your diet (for a 1000 calorie deficit per day) you will lose 2 pounds per week. Losing 1-2 pounds per week is the safest way to lose weight and there is a higher chance off keeping the weight off when it is lost at a slower rate.