How long would it take an overweight 15 year old to lose weight? i really need help.?

i just turned 15 a few weeks ago. i am very very overweight and i am going to the beach witht he fam in approx 4-6 weeks and i dont want to go there looking all fat and ugly and stretch marks all over my stomach. i am getting out and working out everyday until i go but how much can i apporximately lose in the time i have? if someone could help me it would mean so much toi me it is all i can think about and i reallly need help .thanks