Here is a simple way to make a good marinara sauce. There are as many was to make marinara sauce as there are hungry Italians-Americans. It would be blasphemous to use a jarred marinara sauce.

6 tablespoons extra-virgin olive oil
¼ cup butter
3 large garlic cloves
Handful fresh Italian flat leaf parsley sprigs (leaves only)
½ teaspoon freshly ground black pepper
½ teaspoon salt
Three large cans peeled, diced & stewed plum tomatoes (about six cups)
1 tablespoon dried oregano
2 heaping tablespoons tomato paste

Combine olive oil and butter in a saucepan and heat. Chop garlic and parsley together and add to the pan. Cook slowly for five minutes, then add the salt and pepper. Drain the canned tomatoes and add them along with the oregano to the sauce and cook slowly for 30 minutes. Add the tomato paste, stir well and remove from heat. At the end of the cooking, taste for salt and add some if necessary.