Okay so first you have to figure out your body type. Your going to be either a ectomorph, mesomorph, or endomorph. To figure it out, you have to measure you bust(usually your bra size, ex. 32 inches, 34 inches, 36 inches..) your waist(the smallest point along your tummy,), and your hip size.(the widest part of your hips.).
Characteristics of Endomorphs:
small joints and bones, long arms and legs, your body is shaped like a ruler(almost straight down), small shoulders, small cheast/butt, low body fat(without excersizing or following low calorie diets), efficient/fast metabolism, difficulty gaining weight.
Characteristics of mesomorphs:(i am a mesomorph)
Naurally lean and muscular, naturally strong, medium-to large joints and bones, broad shoulders, usually an houglassy figure, efficiant metabolism, responds quickly to exercise, you gain fat as easyly as you lose it. mesomorphs are very atheletic.
Characteristic of endomorphs:
have a smooth and roud body, medium to large joints and bones(usually large), small shoulders, short limbs, most of the time have a higher leval of body fat, body fat tends to settle in the lower regions of body, mainyly lower abodomen, butt, thighs, hips(rather than being distributed evenly throughout the body), usually a pear shaped body(smaller bust and bigger butt and hips), lose weight slowly, slow metabolism rate, attacks of tiredness, difficult to keep lost body fat off. VERY curvy.
I am 13 years old too, 1 weigh 131 pounds, am 5'4, and am a mesomorph. I'm not overweight, put i tend to weigh more because i most born with a large rib cage. you could have that too, it doesn't mean your a endomorph. there are many tests you can take to determine your figure for terms of fashion,(example what to wear, what looks best on you.) but once you figure out what body type you fall in, there are exercises you can do that work more efficiantly than they do for other body types. Endomorphs uaually have to work harder to loose weight than mesomorphs and ectomorphs do(ectmorph usually have truoble gaining....) you could also try eating foods that booast your metabolism. figure out what foods you like and dislike that are healthier, then maybe come up with a diet log. write every food down you eat and the calories content. are you could be mean to yourself like i did because i wanted to stop biting me nails, so i tried not to look or think about them or i would TRY to think about how pretty other girl's nails were. :D i now have long nails. Hope this helps, but I really think you are fine the way you are. :)
try this quiz: http://www.shopyourshape.com/calculate-your-body-shape.html
http://www.superskinnyme.com/body_type_calculator.html(info about 3 body types) there are also nutritional plans and exercise plans for each.
all body SHAPES: http://www.shopyourshape.com/body-shapes.html