I don't remember that guy's name, but he is fairly famous. He did a lot of research and came to a conclusion. He puts it in 7 words:
Eat food. Not too much. Mostly plants.
The 'eat food' part means try not to eat too many chemicals in your food, basically 'don't eat anything your grandma wouldn't recognise'. Not too much - don't overeat, humans need just a few handfuls of food a day, look at the Japanese, how small sushi is
Mostly plants - the vegetarian-er, the better. Vegan would be best.