Obviously calories aren't everything in terms of diet and health; vitamins, nutrients and other parts come into play. But when it comes down to actual physical weight, nothing else matters, unless you have a medical problem. Learning how calories work and counting them for some time, so you understand what you are actually eating, is the first step.
Ideally this work would be done with a professional nutritionist, since calorie counting will get you to the weight you want, but it speaks nothing of how healthy your diet is. Sadly this is not something everyone is privileged enough to be able to do.
It is also important to remember that most people who struggle with their weight, and are making an effort, are actually struggling with their relationship with food. This is something that no matter what diet you are on, or how deep your understanding of biology and nutrition is, nothing can replace actually working that deeper issue, which is not trivial and something truly personal.