I need sum tips to loose my fat?

i am 16 years old...and im tall abut 6 feet almost...and i hav sum fat growing on my tummy....wot shud i do ? i used to wear small sized shirts now within one month i wear Large wot shud i do...shud i join gym ? but ppl say if u do gym and u leav it ull grow fat wot shud i do????? if u guys hav a msn id den z.s.000@hotmail.com is ma msn id plz help cuz im r eally worried abut my body :(