A good mix of foods, and not too much junk food (sorry!). Or too much processed rubbish (which is loaded with fat and salt). Or too many fizzy sugar-y drinks.
People talk an awful lot of rubbish about diet, and diets, but basically the best advice is to eat regular meals, eat a reasonable mix of foods (most people have good idea what food is healthy and not so healthy), and don’t eat too much. One famous line we sometimes use for diet advice when people ask is:
I agree with Austin: variety is important.
Calorie requirements are based on how active you are: if you don’t do much exercise at all, then a teenager needs about 1700-1800 calories a day to maintain weight. If you’re *very* active then you need more, around 2300-2400. Note that there isn’t a massive difference here – 600 calories is about what you’d get in two shop-bought sandwiches – so it can be easy to eat more than you need.
A mix of protein, carbohydrate & fat (of different types), with plenty of vitamins is what is important, so fruit, veg, meat, fish and dairy, with some cereals should form the basis of a good diet. And not too much salt! Most of us eat a lot too much salt, partly because it is ‘hidden’ in processed foods like pizzas and ready meals.
It is also worth staying away from heavily processed foods. I like baking bread at home, for example, as I know what goes into it. The food pyramid works well (try Google) and gives you a good idea of what to eat. One thing that I found a problem as a (chubby) teenager was portion control. Portions are roughly the size of your closed fist. Once I found this out it made things a lot easier!