It’s really hard when other people have so much control over what you eat and stuff, but hang in there! I remember when I was 12, I really wanted to lose weight, too. Everyone saying that you are too young to worry and stuff just doesn’t remember what it’s like. But seriously, if you try to “diet” and lose weight too quickly, it doesn’t work. You have to try your best to develop healthy habits as much as you can—not go on crash diets or exercise for hours a day.
Try to eat as healthy as you can with what your parents pick out—eat more vegetables when they are available, and eat smaller portions of unhealthy foods.
Joining track is GREAT! Even if your parents can’t make it to the meets, you will make new friends, and they will cheer you on. It might take a while for your teammates to accept you into their group of friends, but hang in there. As they see your dedication and improvements, they will become closer.
Good luck!