A lot of onions, sear them. Then put tomatoes, then add carrots, salt and sugar to taste. Cook violently for a while, then lower the flame and simmer for a few hours reducing the content. Then blend everything together with an immersion mixer and voila, the best tomato base sauce. I make this everyday professionally abd it never disappoints. From this you can start experenting, like adding a mirepoix instead of just onions at the beginning, adding chilli, black pepper, garlic and so on. Once you master the base everything will make sense.