The most foundational thing you need to know is that weight loss only occurs when your body is burning more calories than it is eating- on average and over time.
1. Figure out how many calories your body is burning. Use a calculator similar to this one, my favorite.
After you input your statistics by turning the dials, two numbers will calculate at the top of the calculator. The top number is your Basal Metabolic Rate. This number is the amount of calories your body needs to function. This amount of calories keeps your organs living and doing their jobs and is how many calories your body will burn in a single day if you were in a coma.
The second number is your estimated total daily energy expenditure or your TDEE. This is the number of calories it is estimated your unique body burns in a single day while taking into account everything you told the calculator about your lifestyle.
2. Understand that if you eat the same amount of calories as your TDEE each day, you will maintain (no loss or gain) your bodyweight. If you eat MORE calories than your TDEE each day, you will gain weight. If you eat FEWER calories than your TDEE each day, you will lose weight. This is called a calorie deficit.
So it's that simple?
Yes. And no. That's how weight loss works, but there are a lot of other things to keep in mind.
1. A safe and healthy, sustainable calorie deficit is only 300-800 calories per day. That means that eating any less than ~800 calories per day below your TDEE could be unhealthy- physically and mentally. Things get a little tricky here because we tend to do the following:
a. underestimate how many calories we eat
b. overestimate our TDEE
2. Consistency is absolutely key in successful weight loss. It will do you no good if you maintain a consistent calorie deficit throughout the week and then eat all the things on Saturday. See why: Don't Eat All The Things on Saturday.
So I can eat ANYTHING, so long as I'm burning more calories than I eat, and I will lose weight?
However, it's possible that eating less nutrient dense foods could result in a sluggish state, which would actually lower your TDEE, so watch out for that as it would be counterproductive.
On the flip side, note that switching from 2500 calories of Oreos to 2500 calories of veggies and grilled chicken will not result in weight loss, either. See why: I Eat All The Greens and Lose None of The Weight.
What about eating small meals all day long... doesn't it speed up my metabolism (increase my TDEE)?
No. See why: Does How Often I'm Eating Matter?
But!! My trainer said.....
Yeah... that happens. See why: Your Trainer Actually Knows Very Little About Nutrition
But!! I read on the internet.....
I'm gonna let that one slide.
Okay now I'm freakin' overwhelmed. Where do I start?
Start at the beginning. Find out your TDEE. Track your food intake, make sure it's lower than your TDEE on average. If you eat over your TDEE, don't hate yourself for it. Give yourself a break sometimes. Eat the things you like. You can eat your favorite foods and still lose weight. Surround yourself with a support system- you will need them. Think critically about what you hear and read and know that it all, essentially comes down to the science.
You don't need to buy anything to lose weight.