The US Dept of Agriculture has several excellent web site to help us understand nutrition and diet. In the past the food pyramid triangle was the guide for what we should eat to be healthy. Now it's changed to a "choose my plate" graphic. Take a look at:http://www.choosemyplate.gov/
Be sure to check out the other links at the bottom of the page. The "supertracker" link is especially useful for someone on a diet.
The "Food-A-Pedia" is also quite useful. You can find the nutritional value of foods and compare any two. Especially useful when trying to decide between several foods. Ideally you would pick the healthier one with fewer calories.
The USDA info represents the best scientific info available. Since it's not a commercial web site pushing some product or diet plan it's info is more objective than other.
OK now for a little teaser... What's the difference between a food calorie and a physics calorie? A physics calorie is the amount of energy need to raise 1 gram of water 1 deg C. What the hell does this have to do with food? I'll let a few post some guesses or opinions. I was curious and found the answer. Along the way, I was surprised how many registered nutritionist I asked didn't know. To be fair the answer was in their first year text book that they misplaced or long ago forgot about.