What is the most important thing to eat healthily?
Not that this is a problem in developed countries, but the most important thing to consider for proper nutrition is the calories.
We need a minimum of calories a day to sustain life. Food is our fuel and if we don't ingest that minimum, it does not matter how well we eat. This is what we see in poor countries, where food is in short supply and children growth is stunted.
Developed countries, where food is plentiful, have the opposite problem. Too many calories lead to obesity, which is not healthy.
So the first consideration is to eat the right number of calories and, if in doubt, to have slightly less, though never less than your basal metabolism. The latest research shows that people with frugal eating habits live longer.
Once considered the minimum amount of calories, you may think about their quality: less than a third should come from fat, and you can decide how many you would get from fruits, vegetables and sources of protein.