TeenHealthFX is concerned with why you would like to know exactly what your body fat is. If you are not getting your period, it is extremely important that you see a doctor or a nutritionist.
It sounds as if you have a problem with your weight and it may be too low. Everyone's body needs a certain fat percentage in order to carry out essential life functions, one of which for girls is getting their period.
TeenHealthFX recommends that you see a doctor who can help you with this. A doctor may refer you to a nutritionist who can give you information on a healthy weight range for you and discuss proper eating and exercising habits. While waiting to see the doctor, eat healthy and exercise regularly (3 - 5 times per week).