Here in Episode 1, we'll try to clear some basic terms and doubts.
The Calorie (Cal): "the energy needed to raise the temperature of 1 kilogram of water through 1 °C, equal to one thousand small calories and often used to measure the energy value of foods". In basic terms, it's just mean energy. When you move at all, you need energy, when you eat any food, they contain energy, hence all this is the basic measurement of energy.
Basal Metabolism: The amount of energy a person requires for basic functions (breathing, blood circulation, basic organ function) at total rest. For a regular person, additional of a person's life style is important too. A small sized sedentary person might require just 1200 Cal, while a big active person might require up in excess of 3000 Cal.
The important equation:
Caloric intake - Basal Metabolism = Net excess or deficit
It is a simple or as difficult as the above if a person wants to increase or decrease weight! Basal Metabolism can be measured by a few various ways and also via a few different machines. A type of calculator is found here. https://www.muscleforlife.com/bmr-calculator/. Such methods are not 100%, a survey of the various methods has yielded around a 15% variation. Hence a ball park figure is probably what you might get.
The difficulty increases quickly with the tedious nature of measuring or estimating the amount of calories one is consuming. Professional body builders and athletes measure every gram of their intake for peak performance. For the rest of us, we grapple at the amount of calories of a 'medium sized steak'. One popular free app-based calculator is http://www.myfitnesspal.com/recipe/calculator. It gets to pretty accurate within 10-15% of what you are consuming.
The difficulty will increase further, with is a fat calorie the same as a protein calorie? Dozens more confusing and conflicting topics floats in cyberspace, perpetuating this multi billion dollar industry.