Unless you have some (imho rare, <=5% of people) genetic tendency to being fat, this should work:
- eating more vegetables (almost as much as you want of them, lol)
- less obviously bad foods (e.g. cheap cuts of meat where you didnt even cut off anything so you literally eat pure fat with some bites). This doesnt mean you need to go vegetarian
- avoid tons of oils, butter or the likes (not none, but they have insane energy density, just compare (wikipedia):
- Olive oil: Food energy per 100 g (3.5 oz) 3,700 kJ (880 kcal) (https://en.wikipedia.org/wiki/Olive_oil)
- Broccoli: Nutritional value per 100 g (3.5 oz) Energy 141 kJ (34 kcal) (https://en.wikipedia.org/wiki/Broccoli)
And also note that due to olive oil being a liquid and broccoli spread out and having lots of air between, the comparable volume of olive oil per broccoli is completely absurd.
- Physical activity. E.g. jogging, but anything you want. Do Yoga, go bouldering, play tennis, golf, do freeletics, walk up a mountain, whatever makes fun. If you hate doing sports, you won't do it on the long run. It doesnt need to be tons of hardcore training sessions, just physical activity.
- Sleeping properly. Yes, sleeping is important.
To be precise, studies show that people neglecting 2/3 of the three of (proper eating/physical activity/sleeping) have a considerably increased chance of being/getting overweight. Last but least, read studies about the matter from distinguished sources. It's not like noone knows what to do against obesity, it's just that noone cares (especially not the food industry)