It definitely depends on what kind of diet you are following. To lose weight, it is best to limit the amount of carbohydrates (bread, pastry, anything sweet) you are consuming to a very small amount. Best foods for weight loss are proteins (meat, eggs (whites especially), fish, seafood), greens and vegetables (zucchini, cabbage, tomatoes, cucumbers, etc.).
To just follow a healthy diet, eat greens, vegetables, grains, soups, beans and stay away from sweets except for fruit.
In any case, drinking at least 1,5 liters of water (water, not coffee or juice!) a day is crucial for any diet.
A person should follow a healthy diet plan. This should include small portions of proteins, carbohydrates and sugars. Eat a lot of fresh fruits and vegetable as well as drink a lot of water. Moderate exercise is recommended.