I'm 15 years old, 5'6" and I weigh 140, and want to be 120. Would that be healthy?
Many experts consider it to be impossible to lose fat from a single part of your body (called spot reduction). You can't only "lose belly fat". You have to reduce your overall body fat percentage, and replace the lost weight with muscle (because losing enough weight to see a significant reduction in "belly fat" will put you at significant health risks).
There are many ways to reduce body-fat percentage. All of them involve a healthy, balanced diet and regular exercise. One good resource is The 4-Hour Body; the author recommends eating a diet with minimal grains and sugar. He also prescribes a stripped-down exercise routine that (he claims) only takes 4 hours of gym time per month. This is not the only way to lose body fat but, in my opinion, it's one of the simplest, and keeping it simple is very important when you are starting something new.