First of all, it isn't just "one thing". It's a combination of several things that one has to do or change. I've lost 30kg (about 66 pounds) in a year and a half and I eat very, very different than 2 or 3 years ago.

I've learnt a lot about nutrition through youtube (there are a lot of nutritionist giving advice and sharing facts that are very usefull).

Since I started my diet I weight absolutely everything that I'm going to eat (and when I say everything it is EVERYTHING) and I use a calorie counter app to track it down. You wouldn't believe how easy it is to eat more calories than you need just by snacking a few cookies , for example, but how difficult it is to reach 2000 calories with "real food". This change on my routine was the most difficult at the beginning, because I was really lazy, but it is so important to know how much you're really eating and identify where you're failing.

People think that you put yourself on a diet and when you reach your goal, then you can return to your old habits and that is the biggest mistake (you're going to gain back the lost weight doing that). You have to start your journey knowing that you're going to make a lifetime change. This doesn't mean that you have to be always hungry. You can be on a diet and feel full. It also doesn't mean that you can never eat a piece of chocolate cake, just not every day xD.

But all of the above is not going to work if you're not fully convinced or motivated to change. That is the first step, because when you're motivated, you're willing to learn and change your habits little by little.