The best diet is one that incorporates lots of fresh/frozen veggies, lean protein such as meat, eggs, dairy and legumes, healthy fats such as avocados, olive and coconut oils, nuts and seeds, along with a serving or two of fruit/day.
Most vegetables have so few calories that you can eat as much as you want of them, without worrying about eating too much. Keep your meat servings to about the size of a deck of cards. Add olive or coconut oil for seasoning or to cook with. The fat will help make you fuller and will keep you satisfied longer. Limit fruit to one or two servings/day, as it has a lot of sugar and even though it's "natural" it can still cause weight gain.
Eliminate sodas and other sweet drinks (including diet drinks) and make water your drink of choice and exchange white bread and regular pasta for the whole grain varieties. Use brown rice instead of white rice.
Make sure you get some exercise since exercise builds muscle and muscle burns fat. Any type of exercise will do, as long as it's something you enjoy and will stick with.
Limit your carbs if it's medically possible - when you do eat carbs, do it in veggies, fruits, and brown rice.
Look up recipes from nutritionists and make those.
Start taking walks as often as u can.
Pick your healthy weight add a zero and eat that much (example, if you want to weigh 150lbs add a zero 1500 calories, and you shrink to that weight. Filling that calorie amount with healthy food is best though.