I may look thin but im not?

ive had this problem for a while. when i was in sixth grade i was fat, and i lost a lot of weight going into 7th but i also got taller, then in 8th i was taller but gained a little, im in 9th grade now and still have this problem. with my shirt on i may look thin/or average but im accually really flabby, its so embarring because if people ever call me thin, i know im not. i have rolls and my stomach is kind of wrinkly. its just awkward. i guess i need to get "toned" but does anyone know any exercises that could help me? (i think some of it might be extra skin from weight loss, but im not sure)