Well mainly its because of your hormones playing right now, because 13 is the age of puberty and the start of having such low-esteem thoughts.
To keep yourself healthy & well maintained,
drink plenty of water
have your 3 meals and snacks in between (don't ever miss out on breakfast)
and exercise by playing a game or doing something thats physically challenging but fun also
and those beauty magazines are pathetic.
did you know half of models aren't even healthy? models who have such large legs compared to the rest of the average girls are the abnormal ones...so really i don't know what you're worry about :P
and again those beauty pageants, they are nothing without make-up etc.
But don't go powedering yourself with cosmetics, actually stay light with cosmetics as you are very young and dnt want your skin to get poisoned with such chemicals at a young age.
Keep to the basics, take care of yourself and trust me, playing sports, and getting a bit of lean muscle here and there will definitely improve your self-esteem. Stay healthy and your body will feel blessed and light bcos you havent killed it with horrible diets etc.