Philosophy

Macronutrients are dietary carbohydrates, protein, and fat.

The following are basic guidelines, and your needs will depend on body type and what type of activity you are doing, whether it's basketball, cross country, bodybuilding, football, etc...

40-50% of your calories should come from carbs. There are 4 calories in 1 gram of carbs. Shoot for higher carbs if you're doing sports like track or cross country.
30-40% protein - 4 calories per gram. 12-20%, which is recommended for the typical person, is inadequate for active people.
20% fat - Fat has 9 calories per gram. Don't go lower than 20 or your testosterone levels will drop. By going higher you may add extra calories that could be turned to additional body fat. The guidelines for Americans say 30% fat; the reason for this is that 30% is much lower than a lot of people already eat. This kind of fat intake, combined with other poor eating habits and low level of activity, makes obesity very common in this country.

Of the fat you take in, most of it should be monounsaturated (monounsaturated fats raise testosterone.)
Keep saturated fat to a bare minimum and avoid trans fat, which causes serious health problems.

Keep fiber high for optimal digestion and thermogenesis (when the body produces heat to burn calories). Keep simple sugars low; the best time to have them is soon after a workout. Starches are good for you and are necessary for endurance athletes; the less active you are, the more moderation you must use. But you need to get active, because going on low calorie diets without exercising not effective at all--this is why most diets fail and the fat weight is gained back very quickly in a yo-yo effect.

High protein is a must, but don't eat 45 chicken breasts a day like one Denver Broncos lineman did, or you will turn into a weakling. Protein is key for hypertrophy/muscle mass, all bodily functions (including immunity), and muscle recovery after intense training. Carbs, however, are the main fuel for your workouts.