Losing weight essentially boils down to how many calories you take in vs. how many calories you burn with daily activities and exercise.
There are 3,500 calories in a pound. To lose 1 pound a week, you need a deficit of 500 calories per day, by either eating 500 fewer calories or burning 500 calories through exercise. But first you need to calculate how many calories your body needs a day to maintain your current weight. Your 500 daily calorie deficit is based on that number.