The products you purchase at the store have a label on them called the Nutrition Facts label. Here you will find a great deal of information about the food's nutritional content, including total number of calories per serving. The calories are located just below the thick black line and the words "Amount per serving". It is important that you also note the serving size listing on the label, which is located at the top, just below the "Nutrition Facts" heading.
You can also use Calorie Count Plus to search for the nutritional information for many different types of foods, including restaurant foods, for free.
Additionally, food manufacturers, fast food restaurants, and other restaurant chains often provide the calorie information for their foods posted in the dining room, in a brochure, or on their Web site.