How many calories you need each day to lose weight depends on your activity level (both exercise and the rest of the day). A lb of fat is equal to 3500 calories, so to lose 2 lbs per week, you’ll need to burn 7000 more calories than you take in (roughly 1000 calories per day). Do this through a combination of reducing your calories and exercise. Otherwise, you’ll end up eating so little, you’ll be miserable and unlikely to maintain that diet long enough to see success.