First off, Madison, it really angers me to hear that your friends are calling you fat like that and it's affecting you this way. Friends all mess around with each other, and sometimes it can be in a negative way, but do they know how they are affecting your feelings and making you feel about your body? Because if so, they are not true friends if they continue to do it.
One thing you need to realize is that almost every single person deals with different body issues. Whether they think they're too fat, tall, hairy, big boned, muscular...there's always something that bothers us. Personally, my problem was trying to GAIN weight. I've always been severely undersized, and many people accused me of having an eating disorder (anorexia or bullemia) and because of it, I did develop an eating disorder, of over eating to gain weight.
I guess the moral of the story is to try your hardest not to let other people's views of you affect you too much. Trying to obtain a healthy body type is a wonderful goal, and you should work hard for it, using all the different ideas that have been suggested to you so far, but don't sweat it too much, and don't over do it, because beauty is in the eye of the beholder, and real friends won't treat you the way these people do.
My advice on losing weight? It's all in proportions try to eat more times a day, but to cut down on the calories per sitting...drink a lot of water so that you're stomach is never empty and rumbling...most people don't get the proper serving of water a day...and when you really, really, crave sugary and/or salty processed goods we all love like oreos or potato chips, try only eating the serving size...10-15 chips or 2 cookies, and having something to drink like milk, juice, or water to fill you up. If that's too tedious, try getting your mom or dad to buy 100 calorie packs...they make them for almost any kind of snack anymore, and sometimes they're just what you need when you have that need for a cookie fix.
Good luck with the weight loss, Madison.