First, let’s clear up the difference between weight and body fat. Weight is everything: muscle, fat, water, limbs, organs, everything. Body fat is fat stored beneath the skin and intramuscularly. You can lose weight without losing fat. You can also lose body fat without losing weight. I assume most people mean ‘bodyfat’ when they say ‘body weight’.
Here’s how: Calorie deficit (burning more energy than you eat). This means either 1) burning more calories (extra work) and 2) eat less. The easiest way to add in extra work is to walk everyday: stairs, 10 minute walks, parking farther away from the office, etc. The easiest way to eat less long term is to either 1) track your food or 2) eat calorie-sparse foods (vegetables and lean proteins). Combine these two for a winning effect.
Eat less crap.
Do more work.
The main problem is people will take this way too far and start trying to complete marathon workouts 6 days per week while eating 1200 calories per day. DO NOT DO THIS. Start where you are, track your weight and food, and make small tweaks from there.