I'm 14 years old, about 120 pounds, but I want to be thinner. What's the fastest, best way to do it?

I've had weight problems all of my life. I used to be really chubby, and once I lost the weight I kept going and going. My family took me to a nutritionist, who diagnosed me as an exercise bulimic. After that, my whole routine fell apart. I lost my motivation, but I need it back. I have to be skinny.