All you need to count calories is some basic information such as the food, amount, and the caloric value by the unit of food. While gathering this information may sound difficult, most restaurants and food packaging companies make this very convenient by providing that information in organized charts. For fresh meats and vegetables, counting calories will require either a book listing foods, or some online research to make sure you get as accurate of a count as possible.
The first step is to identify the food as closely as you can. If you are a guest at someone's house or are using an old family recipe, these may be harder to judge. If for example, you are eating lasagna in someone's home, you should be able to use a generic equivalent online or in a calorie counting book. If the recipe came from a book, it may give you accurate calorie information, provided the recipe was followed as listed in the book. Restaurants make it easy to count calories by having that information available, usually upon request.
Once you have determined the food, the next step is to determine the amount. This has gotten easier in recent years, especially at fast food restaurants that not only have chart listing the caloric value of everything on the menus, but also print that information on the food packaging. In other cases, you may use a scale to get a very accurate measurement, or you may be able to judge the approximate amount to get reasonably close.
Third, compare the amount to the caloric value of the serving size. For example, if one apple is 80 calories, and you eat half of it, you must complete that calculation yourself. You would have consumed 40 calories. The amount of foods like ice cream, spaghetti and many others may be harder to judge, but if you have a food scale or measuring equipment you can always use those tools to get the exact amount. Eventually, you will became familiar enough with serving sizes that you can use your own judgment.
Also, remember that to accurately count calories you have to remember to include any additional items you put on your food. If you are eating non-sweetened cereal and you add sugar, then you must count calories that are in the sugar, as well as in the cereal. The same is true for spaghetti and sauce. The caloric value on the spaghetti does not include any sauce you may decide to use. If you forget to do this, your calorie count could be way off.
Some people also wish to keep track of their calories from fat when attempting to count calories. While this may be an indication of how healthy you are eating, it may make very little difference in regards to weight loss. If calories are being used, then the body will eventually take fat and use it for fuel when it is needed, so fat calories also eventually get used. Still, if you are concerned about fat content, counting fat calories is a good way to help you keep track of your intake.