A calorie is actually calculated by the amount of energy that is required to raise the temperature of 1 gram of water 1 degree Centigrade. Calories are usually used to describe the energy content of food, which is technically a kilocalorie (1,000 actual calories). Since different foods provide different amounts of energy, this would explain why a small piece of chocolate would contain more calories than a serving of vegetables. You will also need to consider what your personal daily caloric need is. Many calculators can help you determine this.
Consult a nutrition guide such as NutritionData.com to research the caloric value of ingredients for your serving of food. NutritionData also includes popular restaurant and fast-food items.
Choose the food or ingredients from the list. You can search for food items or browse food categories. If you are going to eat an 8 oz. serving of a pot of stew, for example, you are going to need to search for every ingredient--meats, vegetables, butter, flour, stock--going into the stew.
Choose the serving size that pertains to you for each ingredient. If there is a label on any ingredients you are using (such as a can of vegetables), there will most likely be nutritional data listed somewhere on the packaging. Search a nutrition guide for the remaining ingredients. Write down the calories, if you are searching for multiple ingredients.
Add together the calorie total for all of the ingredients. If you happened to search for a full meal, then divide the answer. For example, if you were making stew and added together the total ingredients for the whole pot, divide it by the number of servings you want to get out of it. So if you were to divide 2,000 total calories by four servings, you would get a total of 500 calories per serving.