To take off 1 pound per week, you need to create a "deficit" of 500 calories per day. You can do this by eating 250 fewer calories a day (for example, cut out a 20-ounce bottle of regular soda) and burning an extra 250 calories through physical activity (for example, walk for 2.5 miles). Another way to cut back on calories is to watch your portion sizes.
Here is a calculator to determine how many calories you need a day to maintain your current weight depending on your physical activity.http://www.cancer.org/docroot/ped/content/ped_6_1x_calorie_calculator.asp
I know that it seems unusual that the American Cancer Society would be discussing healthy weight loss. The reason is that obesity is a risk factor for cancer especially colon. Obesity has been linked to breast cancer, uterine cancer and esophageal cancer. Although debatable, there may also be a link to prostate cancer.