Fitness for the summer help? *details?

Hi my name is Mkenzie Linnea, recentaly ive been attending my moms cardio exciresize. ( im 13) .. i want to get in shape, but i feel like im not, i look fine, my stomach is flat.. im 105lbs, and 5'4 Im verry active, i do dance, soft ball, gymnastics, cheer and volleyball in the summer.. adding on to that i take acting and voice, one question-- how can i boost up my confidence, when im always slouching about my best friend ( cousin) who is my same age, and 5'7ish and the same weight! she is a twig