How much should a 5'6 13 year old girl weigh?

Okay, so i'm 13 years old (girl, b-day March 20), 5'6, big boned (like im not a skinny little twig), and I weigh 120 lbs. My arms only look fat when I press on them and my legs kind of jiggle when I walk. My stomach...well, let's just call it a huge glob of fat. My face is round but not exactly fat..but I do have a pretty decent amount of neck fat. I used to take tennis and tae kwon do, but it got too expensive so I quit. Other than that I don't really work out alot. My family and friends tell me i'm just the right size for my age and I usually believe them.....but then when I go to school or shopping or something, i'm surrounded by all these little skinny people and I feel fat. Am I overweight? Should I work out more or go on a diet? I need some serious help :(