Stay away from...
#1 Highly processed foods. Try to eat as natural as possible. The more food you can grow yourself or buy from a local farmer the better. Or if you live west of the Mississippi river you make look into "Azure Standard."
#2 Carbohydrates. Pasta, bread, potatoes, cereal, corn, etc.
#3 Sugar. This one is way worse than many people realize.
And remember, NATURAL FAT IS OKAY! Whole milk, cheese, butter, and lard are way less fattening than fast food, highly processed food, sugar, and carbohydrates. I eat a LOT of dairy and still stay slender. We have three milk cows so we always have a surplus of dairy products.