Sigh. This thread could easily degenerate into a flame war because many people have conflicting beliefs about the answer here. Yes portion control is important too, but you asked about diet.
The standard answer to your question is: follow a low-fat diet. Lean meats, oily cold water fish, vegetables and legumes, whole grains, low-fat dairy and certain oils (sunflower and olive, I believe?).
The less-standard but becoming-more-popular answer is: the low-fat diet is terrible and it is responsible for the obesity epidemic. To lose fat and be healthy, eat low-carbohydrate and high fat, while trying to avoid eating huge amounts of saturated fat. Eat lots of vegetables, too.
Regarding fruits, they contain nutrients but they also have high levels of natural sugar (bad) and they contain fructose, which apparently even in its natural form can screw you up. So fruit diet…not so much.
A wise person once said: “Eat food. Not too much. Mostly plants.” ...and I think that’s great advice. The best thing you can do for yourself is to try to choose water, coffee or tea (with very little sugar) instead of any other drink. At the same time, drastically reduce the processed food you eat, like TV dinners, and cook from scratch without eating much pasta. Throw in a small amount of moderate exercise (30 mins a day, say) and you will be set.