It is not about calories or fat. There is a lot of misinformation out there.
It is about portion control, not eating more than you need, and avoiding empty calories.
The FDA website has a graphic that shows what portion size you should eat. For example, meat/protein should be about the size of a deck of cards.
Empty calories are things like alcohol which have a lot of calories but almost zero nutritional value.
Things like potato chips and other snacks that are high in fat content, and low in vitamins, minerals, and fiber that you need for nutrition. Stay away from artificial sweeteners, like NutraSweet, stevia, etc. Refined sugar is bad enough for us, but these artificial sweeteners are ten times worse.
Everything in moderation and balance.
There is nothing wrong with a burger, fries, and a non-diet soda once in a while, but not every day. And not a half-pound burger either. Portion control.