Sometimes I realize that you Americans mean, with the same word, something that's not exactly what I mean...so, I'm not sure about what a "Tomato sauce" is for you.
1)Fresh plain tomato sauce (Salsa di Pomodoro)?
2)Fresh pasta sauce made with tomato and other ingredients (Sugo di Pomodoro)?
3)Preserved tomato sauce, to be kept for a long time in sterylized glass containers (Conserva di Pomodoro)?
Assuming you mean something between 1) and 2), those are some advices from Mamma:;)
1)You can use both canned and fresh tomatoes, but of course fresh are better. Try to find San Marzano or at least plum tomatoes, that are the best for sauce as they're particularly fleshy.
2)If you have time, blanch them for a couple of mins in boiling water to peel them easily. You can also cook them unpeeled, but the result is less good.
3)The other ingredients are not so important as you can think. Every italian cook has his own recipe! Add garlic, chopped onion, herbs, carrot and celery (chopped or in a wholepiece, in this case to be removed when the sauce is ready), pepper or chili pepper, salt, a pinch of white sugar...try also with a piece of lemon peel!
4)The secret is the cooking procedure. Be patient! A good Italian sauce requires alot of time. Fry very gently the chopped vegetables for AT LEAST 10 mins (until very soft) in good olive oil (possibly Extravergine Olive Oil. The quality of the oil is VERY important);
5)Add the chopped tomatoes, season with salt, pepper and herbs, bring to the boil, lower the heat, cover the pan and leave the sauce quietly bubbling until very thick and tasty. You'll need at least 45 mins or more. When the oil starts coming on the surface of the sauce, it's done.
This is the traditional Sugo di Pomodoro. If you like more a lighter, "nouvelle cuisine" type, you can fry the whole garlic cloves in oil, add the chopped fresh tomatoes, sautè them at brisk heat for 5-10 mins, add fresh basil leaves and it's done!
Let me know if you also need the recipe of the Conserva di Pomodoro...