Calorie needs to Lose/gain weight

Having more calories from food, than you need to maintain your weight can lead to obesity. If food is consumed in excess of requirements, the excess is converted into fat and stored in the adipose tissues. Normally adipose tissues constitutes about 10 to 15% of the body weight. In obese people, it increases upto 30%. 

One can assess if one is over weight, by knowing your required body weight and estimation of the total body fat. If your body weight is 40- 50 % in excess of the required weight you are severely obese. Your total body fat should be less and no more than 30% of the overall diet and about 50% of that fat itself should be vegetable oils rich in Essential fatty acids (EFA). 

Obesity can lead to several complications - metabolic disorders such as Diabetes, atherosclerosis, cardiac disorders; low life expectancy, physical disability etc.  

In order to loose weight you need to reduce your calorie intake but only slightly below your maintenance level. Alternatively, You can keep your calorie - intake the usual and increase your activity level. You can raise your metabolic rate (the basic rate at which you burn calories) by increasing your exercise. So if you want to lose fat, maintain your energy level and improve health, get active and take regular exercise. Aerobic exercise burns a lot of calories. Exercise combined with eating fewer calories promotes weight loss.

A reducing diet should normally provide only about half the daily calorie requirements. But remember that a minimum amount of at least 1200 calories of nutritious food is needed for an adult. Fewer than that would lower iron level and actually slow down metabolism. The goal is to get maximum nutrition with minimum calories. So increase your intake of low calorie, health enhancing fresh fruits, vegetables, whole grains, beans etc and avoid excessive and frequent eating of foods rich in calories and poor in nutrients like fried and fatty foods,  sweets and nuts. Eat fish and poultry instead of red meat, limit your egg intake to four per week, include adequate fiber in your diet. Use unsaturated oils such as corn, sunflower, safflower etc as the cooking medium and cut down on your overall salt intake. Take a multivitamin tablet along with your reduced diet

The rule should be to maintain your body weight constant at the normal level by adjusting the calorie intake. Here is the distribution of food stuff in two calorie reducing diets.

Similarly, in order to gain weight, You should increase your calorie intake above your weight maintenance level. To gain body  weight, you must consume more calories than you burn.