Ideal Weight For Teenage Girl?

What is the ideal weight for a 5'8 teenage girl? In the winter I weighed 135. But I know weigh between 120-125 depending on what I eat each day. You can see my ribs, hipbones, and shoulder blades. I have a eating disorder. But am I at my ideal weight or do I need to loose a few pounds???? HELP! I'll give a high rating to the best answer!