I need to lose inches in my waist fast. HELP PLEASEEEE?

im 16 and live in nj. can i go to rite aid and buy like dexatrim or something to help speed up my metabolism? it wouldnt be long term. only until i lost the three inches on my waist. i only have like 3 weeks. i wana get my belly on pierced so i wana tone up and lose a little before hand. do you think i could just walk in and buy a dietary supplement? im not sure. please helppppp! thanks:)