Making your own marinara could not be easier. Its such a versatile product, too. You can use it for any pasta dish, a soup base, or just for dipping bread. I made it for lasagna. I'll post that recipe later.
Heat some olive oil in a medium sauce pan and saute some minced garlic and minced onions until they are soft and translucent.
Then add the tomatoes. You can use fresh Roma tomatoes (those seem to have the right flavor and texture for marinara) or canned tomatoes. I used some tomatoes that I canned from my garden last summer. Either open the cans or chop them up fresh.