It is generally accepted that one needs, at minimum, at least 1500-2000 calories a day to maintain a healthy body.
Where you get these calories, however, makes all the difference to how your body responds to them.
For instance, did you know that a single gram of carbohydrates or protein is a total of four calories, but a single gram of energy-rich fat is nine calories? On top of the actual calorie numbers that you have to worry about, you must also ensure that the type of calories that you are taking in are the proper ones to maintain a healthy body.
It's true that the body needs hundreds of calories of carbohydrates per day; this isn't to say that you should sit down and eat a quarter-pound of sugar! It's best to have a solid mix of complex carbohydrates from a variety of sources - fruits, vegetables and whole grains are a good start - along with a mix of proteins from meats, fish and dairy if you're interested in eating dairy products.
When it comes to fats, there are also a number of healthy fats that you want to have in your diet as your body needs these fats to keep you healthy.
Staying away from the saturated, useless fats that are prevalent in fast and processed foods is a good idea.
Try to aim for fats such as extra-virgin oils, non-hydrogenated oils, and fats from fatty foods such as avocado, fish, coconut, and nuts like almonds.
If you are aiming to lose weight, you will need to ensure that your body is well-fed to maintain your exercise program, but also not so well fed that you are counteracting your exercise by consuming too many calories.
If you can eat a minimum of 150 grams of protein, 50 grams of healthy fats and 200 grams of carbohydrates from healthy sources (no white flour!!), then you'll be right around 1800-1900 calories for the day, and because of the high protein content in your meals, you'll likely remain satisfied and rarely hungry or needing a snack.
Good luck with your weight loss...
now get out there and shed those pounds!