Diet is arguably the most controllable way to improve health and increase lifespan. There are many aspects of diet to dissect, but a basic start is understanding the importance of macronutrients. The three macronutrients are fats, proteins, and carbohydrates.  Dietary restriction (DR) is proven to be a tool for increasing lifespan.  Instead of restricting calories which can cause malnutrition, the key to a healthy diet that promotes a longer life is balance of protein to non-protein energy.  In short, one can meet this healthy diet by balancing macronutrients.
Fat is made up of glycerol and fatty acids.  The purpose of fat is to store energy long term, cushion organs, and create hormones. The three types of fats are trans-fat, saturated fat, and unsaturated fat.  Trans-fat is found in products like fried foods and baked goods. Saturated fat is known to increase cholesterol and risk of heart disease. It is commonly found in animal products such as beef, lamb, pork, and dairy.  The American Heart Association recommends that only 5-6% of daily diet is saturated fats. Unsaturated fat is known as a healthy fat and can decrease risk of heart disease.  This can be found in food such as avocados, olives, and nuts.
This macronutrient is composed of amino acids which are the building blocks of protein.  This macro is important for growing muscle, protecting muscle, and repairing tissues.  Foods with protein are meat, poultry, fish, egg, milk, and cheese. It is also possible to get protein without animal products with foods such as beans, lentils, nuts, seeds, and soy. Additionally, lower amounts of protein can be found in grains, vegetables, and fruits. 
Carbs are simple sugars composed of either monosaccharides or starch.  Carbs are broken down into glucose, an energy source for the body.  For a basic understanding, carbs can be broken into 2 groups, simple and complex. Simple carbs are easy to break down and are usually sweet, and since they only have 1-2 sugar molecules, simple carbs can be used for short term energy.  Complex carbs take more time to break down and, therefore, they make you feel full longer. Less sweet than simple carbs, complex carbs are foods like rice, pasta, and bread.