Hi There! You've been given some great advice above. I'd just like to say that I think some of the best adivce is not to "go on a diet", they don't last as they are way too easy to come off of ... I know, because I started the whole "diet" thing way back in my teens, and look where it got me! I realise now that the best thing to do is to change my lifestyle/habits ... be more active and choose to eat healthier foods ... plenty of fruit & veggies, pasta, chicken, fish, lean meat, etc., instead of chips, cakes, cookies, burgers, etc. I know it's not easy, but just do it a bit at a time, make small changes each week.
As far as those girls are concerned, try not to take it to heart whatever it was they said (I know that's easy for me to say) ... Unfortunately, it's a fact of life that teenagers (especially teenage girls!) can be such little b*tch*s ... if it's not someone's weight they're talking about, it would be someone's hair, or makeup, or clothes, or whatever, and if it wasn't you they were talking about, it would be someone else ... I remember, I used to be a teenager myself, and I have two teenagers of my own now, so I've heard it all before.
Talk to your mom and let her know how you're feeling ... I guess she's the one who provides you with your meals, so let her know that you want to lead a healthier lifestyle and I'm sure she'll be more than happy to help you. And don't forget you can always come here for support and encouragement whenever you need it. Take Care
