B.M.I. (body mass index):
your weight in kg divided by your height in cm. A range of 20-25 is considered healthy. 25-30 is considered overweight. Over 30 is considered obese. Some people can have a high B.M.I. and not be overweight (like body builders) since muscle weighs more than fat. A better indicator is your waist circumference, or better yet, your body fat percentage (should be 20-25% for females, less than this for males).
B.M.R. (basal metabolic rate):
the number of calories your body burns just to stay alive without doing any other activities. This would be the number of calories you burned if you were sick in bed all day. A rough estimate can be given from www.caloriesperhour.com (also in link above). It's based on your sex, age, height, and weight. You can then determine how many calories you need to stay at your current weight based on your activity level. If you want to lose weight, you need to eat less than this number (200-500 calories less a day). Remember, 3500 calories = 1 pound. Creating a 500 calorie deficit through diet and/or exercise will cause a 1 lb. loss a week. You shouldn't plan on losing more than 1-2 pounds a week, and this is pretty ambitious for long periods. It's very simple: if calories in is more than calories out, you gain weight.