I'm surprised that exercise hasn't been stressed more in this thread. For younger people, with exercise alone without a major diet you can lose tons of weight. It worked for me. My metabolism completely changed once I started working out, and I did not even need to be so careful about what I was eating. I lost over 50 lb. Of course, you cannot just stuff you face with whatever you want, but as long as you eat like a sane person, putting a limit on junk food, the exercise itself will work wonders.
Plus, exercising makes you feel so much more energetic. It's worth it just for that, even if you're skinny. Plus, it's good for your heart and long term health.
Personally, I do muscle-strengthening weight lifting together with cardio. Not body-building stuff, just working out muscles to keep them strong and healthy.
Good luck!