Take a look at this series of pictures. What the World Eats, Part I - Photo Essays - TIME Click through until you get to the American family. Notice all the sodas and juices stacked in the back, all the stuff in packages. And yet, they look pretty healthy.
I can see your dilemma. How can someone who is skinny and healthy figure out what is right when it's possible to eat a diet of mostly artificial food and stay healthy? What you can do is make the best choices you can. The really sick and unhealthy teens are eating worse food than that family. They never have fresh meat or vegetables. They drink 32oz sodas every day. They pop pre-made stuff in the microwave for dinner. They eat too many calories and eat candy and potato chips every day.
Even if you went out for pizza every day with your friends, you probably won't do that for your entire life. It's not going to hurt you, not even in the long run.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 180 x 2. Current Deadlift: 230 x 2