This is a topic A LOT of people need help with and to be honest, I don’t think you’ll find a more comprehensive yet plain-language post on this topic anywhere.
I’m glad you’re here!
Let’s start with the basics so we’re on the same page.
Calories (kcals) = Energy.
To lose weight, you need to eat less energy than you are burning. On the flip, if you are trying to put on weight, you need to eat more than you are burning.
Re: Weight loss – The reason: Fat storage is stored energy. With the calorie deficit you get through food and exercise, your body needs to tap into it’s stored energy to support its functions and meet its daily needs. This is how you burn fat.
Calories = protein + carbs + fat.
Alcohol has calories but this post assumes you keep it minimal to 0 as you start your journey so you don’t slow your progress as alcohol does slow the oxidation (metabolism) of protein, carbs, and fat.