What foods should you eat to maintain a healthy diet, and what is the exact amount of food should you eat daily?

Protein & vegetables. A rough guide to determining the amount of calories the average adult should consume daily is about 10x your weight. But using calories alone to gauge intake is not enough. Essentially I have found that small meals of mostly protein and above-ground vegetables maintains ideal body weight, chemistry and hormone levels. Individual genetics, activity, and medical conditions influence this greatly.
Balance, Variety. 1/4 of your plate as protein- lean meats, chicken, fish, eggs, dairy, 1/4 of your plate as complex carbohydrates, whole grains, beans and legumes, 1/4 of your plate as fresh fruit and 1/4 of your plate as fresh vegetables. You need variety, balance and moderation in portion control.
Depends. To lose one pound/week, decrease 500cal/day. The best diet is a low glycemic diet, plant based, lean protein, and lots of veggies and fruits. Drink lots of water, avoid the alcohol and sugary drinks. Exercise 150min/week.