Regardless of your age, sex, or fitness level, good eating habits are the single best thing you can do to improve your health. Regular exercise and taking vitamins are very helpful, but what you put into your body will have a huge effect on what kind of condition it is in.
The best kind of eating habits you can develop are ones that allow you to enjoy what you eat (not a complete feel good diet however), but have a balanced and good diet that doesn't leave you getting too much of one thing and not enough of another. One nutrient that is commonly out of proportion is carbohydrates. Typically people in modernized western cultures have feel good diets that lead to a higher than necessary carb intake, which leads to those carbs being broken down and stored as fat. Carbs are essentially complex sugars, which are burned for energy, unless they are unneeded. Unfortunately most of the foods that taste the best are loaded with carbs. However, they are not altogether bad, but eating too many carbs can cause you to gain weight. Good eating habits and a good diet plan cause your carb intake to be balanced so that excess carbs are not being stored as fat and you actually feel good instead of having a completely carefree feel good diet.
Too much fat and cholesterol intake is also an enemy of good eating habits. In some ways it is better to eat more fat than carbs, since your body has to work harder, burning more energy, to break down fats. Cholesterol is a necessary part of your diet, but too much cholesterol can cause your heart to become clogged, increasing your blood pressure, and causing you to be less healthy and put you at risk for a heart attack.
Although a good diet plan is going to be a little different for everyone, good eating habits should incorporate foods from all the food groups: grains, meats, dairy, and fruits/vegetables. The classic food pyramid says that you should get about six servings of grains, about three servings of fruit, three servings of vegetables, two servings of meat, and two servings from the dairy group. There is also a designated 'other' group that consists of junk food and beverages, like soda, that have little positive nutritional value. The food pyramid indicates that foods from this 'other' group should be used sparingly, and perhaps should not be part of your good diet plan. However, your intake of food from the 'other' group will depend largely on your metabolism and your health goals.