What is a teenager suppose to eat when trying to lose weight?

Good quality. Goal should be slow, steady weight loss based on a lifestyle change, not a quick fix. Accordingly, both quality and quantity of food make a difference. High quality protein, complex carbohydrates, low fat in reasonable amounts. What is reasonable? Variety of online calculators that will give you a reasonable estimate of how many calories you "need", and how many you are actually consuming.
Spark People. Spark people is an excellent website with lots of diet tips and plans, many of them customized to suit your needs. http://www.sparkpeople.com/.