I'm a fit and fairly muscly teenage guy (just finished working out, actually), but I used to be a bit fat, with no self-esteem at all.
Hell, I just took a girl out to lunch out of the blue today. Funny how things change, hey?
All my fat was gone, but I was still super skinny. I only started properly working out the beginning of last year, but due to my pretty awesome genes (my grand-dad is built like a tank)I've come along pretty damn fast.
People just have to find the motivation in themselves, nothing else will work.
EDIT:
I do eat fairly badly though. I have cut out take-away, but I eat a bit too much chocolate and sugar, which I'm cutting down tons lately. Other than that I eat my veggies, fruit and meat, and keep away from fatty foods.