Energy balance comes down to Calories In versus Calories Out (CICO), which means the food we eat (Energy/Calories) is stored in our bodies if we eat more than we use. So we gain weight when we eat more calories than we use through movement & we lose weight if we eat less calories than we use through movement.
Every process in the body requires energy/calories, first we have,
Basal Metabolic Rate (BMR), which is the energy required to keep us alive, ie breathing, pumping our blood, heart beat & other organ function etc……this does not take into account any movement, then we have,
Non-Exercise Activity Thermogenesis (NEAT), which is all movement which is not done through thoughtful action, ie fidgeting , then we have,
Exercise Activity Thermogenesis (EAT), which is purposeful, thoughtful movement & exercise, all of these things require energy/calories from the food we eat.
We all require our own unique amount of energy/calories to function during our days & it is a unique number to each of us as we all have different combinations of weight, height, amount of activity, age & whats called Thermic Effect of Food (TEF), which means how much a piece of food we eat or drink raises our body temperature to metabolize (break down & use for energy or rebuilding certain structures in the body).
So what do we do with this information, or how do we find out this information? Well it’s not an exact science outside of a laboratory, however we can estimate our BMR using a calculator such as the Harris-Benedict calculator & an activity multiplier, which can be found quite easily online, along with numerous other calculators. There is a little math’s to do with these, so a simpler way could be to multiply your weight in kilograms by 24 for men & 22 for women (if your very over-weight you use your goal weight) & then use the activity multiplier. You then need to decide if you want to gain weight (go into a surplus), add 10% to this number & if you want to lose weight (go into a deficit) you subtract 10–20% from this number (I suggest starting out at a 10% deficit). This final number is the amount of calories you may need to eat to reach your goal, I say may need to eat as it is not an exact science, it is only an estimation & this number does change as you move through your goal, but these figures will get you in the ballpark of where you need to be. Next you need to know that the food you eat & drink contains calories in the form of the macro-nutrients protein, carbohydrate &/or fat,
Protein has 4 calories per gram,
Carbohydrate also has 4 calories per gram,
Fat has 9 calories per gram,
Alcohol is also a macro-nutrient & has 7 calories per gram,
You then need to split up these macro-nutrients across your diet, if you aim for 1.2–1.6g of protein per kilogram of bodyweight you can then distribute carbohydrate & fat as you like or prefer to eat, keeping roughly within your estimated calorie allowance.
To track all this you can use an app like MyFitnessPal, you would weigh out the food you eat & input the weight & food into the app. This can be time consuming, however if you do this for approx 2 weeks you can get a very good idea of portion sizes for yourself & will then be in a better place to go on & eye out your portions.
This is a very basic overview, & you need to keep in mind that the number of calories you need to consume will change over time so vigilance is needed to notice the changes in your body & then adjust your food intake accordingly.
As for “the Diet” you should or should not follow, it all comes down to what fits in with your lifestyle & likes/dislikes. There are no diets with special powers or any diets that are inherently better than others, basically if you like to eat that way & you can consistently eat that way without any ill effects & your getting all the macro- & micro-nutrients you need then your on the right path.
Again this is a basic overview to building your diet around you & your lifestyle, try out different macro splits, try out different foods & have fun with it, as there are infinite ways to eat & near infinite foods out there, find what works for you!