im sorry, sweetheart, i have to agree with the others. i think you are too young for this. please consider doing a raw food program for three days or something? i am 5'3" and Science says im supposed to weigh between 110-140. i am 240. many of use here are excessively overweight/obese. you sound h/w proportionate. you dont need to fast. your body is still growing and changing.
with all that said, if you DO decide to fast... please be careful. listen to your body, break the fast if your body calls for it.