Losing weight is theoretically simple. Take in less calories than you burn during the day and you will lose weight. But how do you know how many calories you should be taking in? Well there is a formula involving basil metabolic rate, height, ages, activity levels that is accurate for most people.
You can use one of the many calorie calculators found on the internet. A good one I found is at http://www.cancer.org/docroot/ped/content/ped_6_1x_calorie_calculator.asp You put in your weight, height, age and it will calculate the amount of calories you need to eat to maintain your weight,or to lose weight. You can lose weight by eating fewer calories or exercising more, or any combination of the two!
I like this calculator because it presents a reasonable number of calories. Remember, starving yourself by eating fewer calories than your body needs is not only unhealthy, it is counter productive to losing weight. When you eat fewer calories than you need, your body believes it is starving and actually slows down your metabolism for self defense.