Eat meat and vegetables, nuts and seeds, some fruit, little starch, and no sugar.  Keep intake to levels that will support exercise, but not body fat.

That's about as simple as we can put it.  But, for more details, below I have copied the CrossFit dietary prescription from CrossFit.com.  Make sure to also read the Sane Nutrition for Kids in 150 words at the bottom of the page.

Protein should be lean and varied and account for about 30% of your total caloric load.
Carbohydrates should be predominantly low-glycemic and account for about 40% of your total caloric load.
Fat should be predominantly monounsaturated and account for about 30% of your total caloric load.
Calories should be set at between .7 and 1.0 grams of protein per pound of lean body mass depending on your activity level. The .7 figure is for moderate daily workout loads and the 1.0 figure is for the hardcore athlete.
                   
What Should I Eat?
In plain language, base your diet on garden vegetables, especially greens, lean meats, nuts and seeds, little starch, and no sugar. That's about as simple as we can get. Many have observed that keeping your grocery cart to the perimeter of the grocery store while avoiding the aisles is a great way to protect your health. Food is perishable. The stuff with long shelf life is all suspect. If you follow these simple guidelines you will benefit from nearly all that can be achieved through nutrition.
                   
The Caveman or Paleolithic Model for Nutrition
Modern diets are ill suited for our genetic composition. Evolution has not kept pace with advances in agriculture and food processing resulting in a plague of health problems for modern man. Coronary heart disease, diabetes, cancer, osteoporosis, obesity and psychological dysfunction have all been scientifically linked to a diet too high in refined or processed carbohydrate. Search "Google" for Paleolithic nutrition, or diet. The return is extensive, compelling, and fascinating. The Caveman model is perfectly consistent with the CrossFit prescription.
                   
What Foods Should I Avoid?
Excessive consumption of high-glycemic carbohydrates is the primary culprit in nutritionally caused health problems. High glycemic carbohydrates are those that raise blood sugar too rapidly. They include rice, bread, candy, potato, sweets, sodas, and most processed carbohydrates. Processing can include bleaching, baking, grinding, and refining. Processing of carbohydrates greatly increases their glycemic index, a measure of their propensity to elevate blood sugar.
                   
What is the Problem with High-Glycemic Carbohydrates?
The problem with high-glycemic carbohydrates is that they give an inordinate insulin response. Insulin is an essential hormone for life, yet acute, chronic elevation of insulin leads to hyperinsulinism, which has been positively linked to obesity, elevated cholesterol levels, blood pressure, mood dysfunction and a Pandora's box of disease and disability. Research "hyperinsulinism" on the Internet. There's a gold mine of information pertinent to your health available there. The CrossFit prescription is a low-glycemic diet and consequently severely blunts the insulin response.
                   
Caloric Restriction and Longevity
Current research strongly supports the link between caloric restriction and an increased life expectancy. The incidence of cancers and heart disease sharply decline with a diet that is carefully limited in controlling caloric intake. “Caloric Restriction” is another fruitful area for Internet search. The CrossFit prescription is consistent with this research.
The CrossFit prescription allows a reduced caloric intake and yet still provides ample nutrition for rigorous activity.

The Zone

CrossFit endorses the Zone diet.  Started by Barry Sears, Ph.D., the Zone diet is moderate in protein, carbohydrates, and fat.  Designed to restore proper balance of your hormones (insulin and glucagon), the Zone diet keeps your blood sugar levels under control, thus maintaining your energy levels and allowing your body to burn fat for energy.  

Sane Nutrition for Kids in 150 words.

Our goal with kids isn't to get them on the zone, but to get them to think and make good choices about what they eat. Our goal is to teach them very basic concepts, sugar is bad, protein is good and you need to eat some in every meal. Nuts and seeds are good fats. Eat them, don't avoid them. Pasta, white bread, and white rice are not that good for you, stuff that's red, yellow, blue and found in the fruit and vegetable aisle is good for you. Eat a lot of it.

Look at your plate, make a fist, eat that much meat every meal; turn your hand over and fill it with nuts and seeds, eat that much good fat, fill the rest of your plate with stuff you found in the fruit and vegetable aisle. Fill your plate this way at every meal, don't eat more.

This is from the CrossFit Kids website. It's intended for children, but it's great advice for everyone.  For some people, thinking about nutrition is overwhelming enough to cause them to stop even trying to fix their diet.  If you have kids, you can pass this advice on to them.  That, along with implementing a fun fitness program like CrossFit, will prevent all the problems that accompany childhood inactivity, such as obesity and related health problems.