I have a problem, and it's hard to fix it, losing weight?

Okay so I do have a problem to me i know i do everyone else can deny it, but i don't care, i NEED to lose weight i way to much 138lbs way to much I'm 13, everyone is smaller than me, I'm not trying to be like everyone else I just wanna not be fat, or "over weight" I need to lose atleast 20lbs, my sister is 25 and a size 0 or 1 in pants O.o, i eat healthy, and snacks small portions of meals i don't know when it'll kick in but I need to lose it before freshman year, and GOOD and USEFUL advice..