I would like to start a diet.. But I don't know what is better to eat as breakfast, lunch or dinner
You could start by cutting down on sugar and refined grains, and most liquid calories (like sweetened coffee beverages, soda, most juices...)
Eat a few more veggies in a day. Chop up a carrot and a cucumber and eat em with some dip. Small changes grow.
Take what you normally eat, and make some changes. Like switch whole grain bread for white, or cut out bread altogether, and eat oatmeal,(not the pre sweetened kind), or some other whole unprocessed grain.
Avoid sugar in your coffee, eat a whole fruit instead of juice...eggs are good for protein. Yogurt (plain unsweetened, with blueberries, maybe chia or other seeds & nuts)
If you aren't counting calories, you might consider cutting down on carbs, which is a kind of easy way to limit total calories, if you don't go overboard on fats. Eating natural fats is okay, don't be worried, but fat does have calories, so don't go eating like a pound of almonds instead of some bread.