To lose 1 pound, you must create a deficit of 3,500 calories. To lose one pound in one week, you may consume 500 fewer calories each day. Alternatively, you may burn 500 more calories each day through exercise. To lose 2 pounds each week, the calorie deficit each day should be 1000 calories which can be acheived through any combination of diet and exercise.
IMPORTANT: Nutritionists agree upon 2 pounds being the maximum weekly weightloss that is healthy. DO NOT attempt to lose more.
Also, lowering your caloric intake too greatly will lower your metabolism, decreasing your results. Because of this, a combination and focus on burning calories through exercise is important. For more information on caloric needs, see the post on caloric needs saved in memories.