How does a 12 year old girl lose weight in 2 weeks?

i am also a 12 year old girl that is trying to lose wieght so my grandma said to help me to lose weight iis when your at the super marget look at the back of any type of food you like eating and try to make sure it has low sodium and sugar cause soduim and sugar is what makes you gain the wieght also to lose wiegiht try to eat early so you have time to lose that wieght and like after 5pm try to eat fruits or anything with low soudim and sugar .