Am i overweight? How can i improve my running/breathing? Whats the best way to burn fat on my legs and arms?

im 12 but my birthday is 7.19 im 5'4 weigh about 110lbs. i can like put my thumbs under my ribs and when i pull my hands in my fingers touch over my belly button (i cant explain) and people say im skinny because of that but my legs jiggly and its annoying i need to wear a back brace because of my scoliosis so i barely have time to work out, and when i run i get tired really quick, is it because of my scoliosis or do i need to lose weight? ive played basketball for 3 years, volleyball in 7th grade and i play field hockey now, (im going into 8th grade next year its summer vacation now). please answer all 3(: