Start eating healthier. Eating healthy is really important if you want to be a healthy teen. You don't need to go on a diet, just pay attention to what you're putting in your body.
If you only ever eat sugary breakfast cereals and candy bars you aren't going to be healthy. Depending on your body and what works best for you, you can eat three meals a day or 5-6 mini-meals a day. NEVER skip meals. Try healthy alternatives, instead of having a bowl of ice-cream, opt for a frozen yogurt. Or try having apple slices instead of potato chips.