The advice already given is great, but I have a question: how tall are you?
If you are 3'11" then yes, being 120 is probably too heavy for you. I am guessing, however, that you are the same height as the average teen girl and are probably about average in weight, too.
The media, such as magazines, TV shows, and movies which feature malnourished, skeletal actresses would have you believe otherwise. Keep in mind that many actresses and models starve themselves, have eating disorders, or get lots of surgical help to look the way they do. In addition, make-up, lighting, and airbrushing all make them look like they are perfect. Almost no one this earth looks like that naturally
So, if you are comparing yourself to what you see in magazines or on TV, then stop
In ten or twenty years from now, when you look at pictures of yourself from now, you will probably shake your head and wonder why you ever saw yourself as "fat" when, in fact, you were just a normal sized girl and would kill to be that "fat" again.
But, like Glory said, you probably won't listen to any of this. When I was 15, I thought no one really understood my problems, and I was too stubborn to listen to anyone who was not in my peer group.
However, if you are looking to lose weight the healthy way or are looking just to get more fit, then looking at the various forums on this website will definitely help. Also, if you talk with your parents, maybe they might be willing to help by cooking come health-conscious dinners or otherwise being supportive.
That being said....