As described in the chapter "Understanding and Enjoying Foods" your body constantly requires energy and nutrients to grow and be healthy. But when you get energy you must spend it, too, otherwise the energy will be stored in form of fat cells around your body. Fat cells, in turn, will interfere with your metabolism (the body's process of unlocking and converting the food energy and nutrients) which will lead to health problems over time. (Talk to your science teacher about the two "Laws of Thermodynamics", they're quite fascinating. A great source for information about these and other scientific topics is "The Joy of Science" by Professor Robert Hazen from George Mason University, http://www.gmu.edu/robinson/hazen.htm).
How do you know how much energy you need? That depends on your age, gender, height, weight, metabolism, physical activity, geography, and genetic structure (which you inherited from your parents). That means, each person has different energy needs. However there are guidelines for the "average" person: teenage girls need between 1900 and 2,300 calories each day, teenage boys need between 2,200 and 3,000 calories per day (based on the national guidelines of several countries in North America and Europe).
The most common measures used to calculate whether you eat enough, too little or too much is the Body Mass Index (BMI) and your waist line/weight ratio. The BMI relates your height to your weight through a certain formula. Go to http://www.nhlbisupport.com/bmi/ to calculate your BMI. If your BMI is between 18 and 25, you want to keep it that way. If your BMI is over 30, you are obese and over 40 you are morbidly obese. If you're BMI is under 18, you are underweight and at BMI 14.9 you are close to starvation. Waist size is another indicator of retaining too much energy in your body. A waist too large for your height and weight is considered especially harmful. For girls or women, health risks start at waist size of 31.5 inches and rise substantially at 35 inches. Boys or men should be concerned about 35 inches and 40 inches, respectively. On the other extreme, waist sizes below 20 for an adult size person may be too low (depending on height and body built).