The most important think is "moderation in all things". Don't over eat, and eat a variety of foods. Balance your intake. Eat more fruits and vegetables, whole grains, fat-free or low-fat milk and lean protein. Consume less fats, sugars, refined grains, sodium and dietary cholesterol.
"Calories in and calories out" is a good focus. If you burn more calories than you consume, weight will drop. If you eat more calories than you burn, usually you will gain weight as the body stores the excess.
Sleep is the great cure-all. It is a time when your body recovers, and when you immune system is improved. Even those who slept 6 hours or less who otherwise had no health problems had death rates 1.8 higher than those who slept "normal" hours. The average adult needs about 8 hours of sleep per night.