Almost everybody knows that if your body absorbs more energy (calories) than it burns, you will gain weight. If your body absorbs fewer calories than it burns, you will lose weight. If the numbers are the same you will maintain weight.
Your body burns energy in four ways and they add up to represent your total energy expenditure (TEE) - 1, your basal metabolic rate (BMR) is the amount of energy required for basic bodily functions. You'd burn this if you were in a coma just to keep your body temperature up and your lungs, brain, and heart going. For most people, BMR is about two-thirds(ish) of their total calories burned. 2, your non-exercise activity thermogenesis (NEAT) is the number of calories your body burns being generally active throughout the day. This can include things like walking the dog, getting the mail, walking upstairs, going to the mall etc. In most people, this is the second-largest part of TEE. 3, exercise activity thermogenesis (EAT) is the number of calories burned through formal exercise. 4, the thermal effect of food (TEF) is the amount of energy your body needs to break food down into parts that it can use.
If and only if the total number of calories your body absorbs (which is NOT the same as what you eat, by the way) is less than the total number of calories your body burns in this TEE equation, you will lose weight. So all you need to do is either reduce the number of calories you eat, and/or increase your BMR, and/or increase your NEAT, and/or increase your EAT, and/or increase your TEF and you will be on your way to creating the deficiency that is necessary to burn fat.
Seems simple enough, but there are some important points to consider. You may have heard that not all calories are created equal. The type of food you eat has a direct effect on TEF and an indirect effect on BMR (e.g., not enough protein could lead to a reduction in lean muscle which could lead to a reduction in BMR). The amount of energy that your body needs to break down different types of macronutrients is also an important factor. Protein, for example, does not exist in the body in the same form that we eat it. Our bodies need to go through a process to break up those protein molecules into pieces that it can use. That takes energy. Fats and simple carbohydrates, on the other hand, are easily used by the body and the body does not need to burn much energy breaking those down. I'm not a scientist so I haven't looked at studies but I've found several sources online and it seems that for every 100 calories of fat that we eat our body burns 3-5 calories to break it down,for every 100 calories of carbs we consume our body burns 5-20 calories to break it down (this had the most variability among sources, from 5-10 to 5-25), and for every 100 calories of protein that we eat our body burns about 15-25 calories to break it down.
So, using all of the above here's a hypothetical extreme example. I eat 2000 calories and I meticulously choose lean proteins and complex carbohydrates and maximize the thermal effect of food. Because of my choices, my body uses 400 calories (20%) of energy In order to process and use the energy stored in those macronutrients. The next day, I eat 2000 calories worth of sweet and tasty buttercream. Tons of fat and simple sugars. Because of my choices, my body uses 100 calories (5%) of energy in order to process and use the energy stored in those macronutrients. Same 2000 calories, significantly different expenditure due to TEF. This is a pretty silly example, but it shows that it's possible to increase your total energy expenditure by doing nothing but changing the types of food you eat. Increase protein and complex carbs. Reduce fat and simple sugars.
Beyond that, NEAT is probably the easiest for most people to increase. You don't need to drag yourself to the gym every day to "exercise." Look for opportunities to add small bursts of extra movement throughout the day. Walk to Tim's instead of drive. Take the stairs. Vacuum the house. If possible, break up your workday and go for a 5-minute walk every hour or so. Anything that gets you moving.
The only way that any overweight person will ever shed the fat is by fixing their nutrition. That's the simple truth. Exercise activity thermogenesis is simply not a big enough piece of your total energy expenditure to make up for a poor diet. And forget the word diet, by the way. A diet implies restriction and deprivation and temporary. Something you need to "cheat" from because you hate doing and you plan on stopping once you achieve whatever you want to achieve. Nutrition is a lifestyle change that requires you to form new habits that you enjoy and change the way you think about food long term. Look for simple changes at first. Imagine you had a secret pill that reduced half of the calories in everything you ate. What you would eat in that magical world? Cook it. Eat it. Then find some small changes that you can make. Love eggs and bacon in the morning? Great! Try 3 eggs instead of 4. Or 2 eggs and some egg whites. Or 1 piece of pork bacon and 1 piece of turkey bacon. Make small changes. Huge changes are unsistainable and that's why most people fail on their diets. You simply won't be motivated to continue eating shit you hate by making sudden changes to deprive yourself.
One last point - for me, I found that intermittent fasting helped me keep my calories in in check. I developed a habit of making my 24-oz coffee in the morning and bringing only that with me to work. Sunday meal prep has become a fun habit as well and I prepare my chicken or fish and salads or veggies (cauliflower rice is so tasty) for the entire week and have them in the fridge ready to go when I get home. I also use MyFitnessPal to track my calories and I set a limit of 1950. It's almost become a game to see how many calories I can "save." I usually get to the evening with about 900 or 1000 calories remaining for the day and I'm nevery hungry enough to eat that much, so i'll have some sunflower seeds and popcorn and maybe a yogurt and end up below my target. You need to find what motivates you. - Also, low calorie foods that take a while to eat are a nice trick for me. Sunflower seeds in particular. 80g (in shell) of sunflower seeds is something like 170 calories and I can snack on them for hours. Get your nuts in shells because it takes longer to eat!