Just as eating often is important, so is eating enough. In order to build muscle, and therefore burn more calories (even when you are resting!), you must give your body what it needs each day. There are numerous free websites devoted to finding out your daily calorie allowance. Look it up!
You might be surprised to know that the bigger you are, the more calories you are allowed. The reason is, when you are larger, your body needs more energy to carry itself around, as well as perform other basic functions. This is why step-down programs like Weight Watchers work- you actually start out eating quite a lot of food, lowering your daily allowance slowly, as your weight and calorie needs drop.