You have to use 3500 more calories than you consume in order to lose a pound of fat. Although this seems like a simple formula remember that your body is a thinking organism designed to protect itself A pound of fat represents approximately 3500 calories of stored energy.
If you were to try to reduce your intake by the entire 3500 calories in one day, your body would register some type of alarm and think that there is a state of emergency. Immediately your metabolism would slow down and no weight loss would be achieved.
It's better to spread your weight loss out over a period of a week, so that you aim to reduce your caloric intake by 3500 to 7000 calories per week, resulting in weight loss of one to two pounds per week. It's generally not recommended to try to lose more than two pounds in a week. Attempting to do so may cause health risks, and on top of this you're unlikely to be successful.
In the example of attempting to lose two pounds per week, you can use a basic method of calorie counting to help you accomplish your goal. To do so, you need to figure out how many calories a person of your age, sex, and weight usually needs in a day, subtract 500 from that amount, and follow a diet that provides you with that many calories.
For example, if you would ordinarily need 3000 calories in a day, you would follow a 2500-calorie a day diet. Next, figure out how much exercise a person of your weight would need to do to burn 500 calories per day, and engage in an exercise plan that will help you achieve your goal.
The result is simple: 500 fewer calories consumed and 500 more calories expended equals a 1000 calorie per day deficit, which, over the course of a week adds up to 7000 calories, or two pounds. Although individual results may vary, the bottom line is if your body is consuming fewer calories than it's expending, then weight will be lost.