Your BMI, or body mass index, is calculated with the formula 703(W/H^2), where weight is your weight in pounds and H is your height in inches. This formula is for adults over 20; people who are younger don't conveniently fit into it. The formula is used to calculate healthy weight.
The BMI formula is only meant as an approximate measurement, it isn't an absolute determining metric of obesity. As a rule it can help, but you should always corroborate your findings with a doctor and test for other factors that put you at risk such as high cholesterol levels.
The result of calculating BMI determines, as a rule, whether you are overweight or not. If your BMI is between 20 and 25, you are considered to be within a healthy weight range for your height. Between 25 and 30 is overweight, and anything above 30 is considered obese. Below 20 is considered underweight.
If you believe you have weight-related health issues, talk to your doctor about any results and come up with a plan to change your lifestyle, such as diet and exercise to promote health. Dieting tends to fail because many diets call for abrupt changes that leave the body craving the foods in question.