Marinara is super easy. This is the recipe I use all the time.
2T olive oil
1 medium onion diced
2 cloves garlic
1 cup carrots, diced
2 cans diced tomatoes
fresh or dried basil.
Put the onion and a couple of cloves of minced garlic into the bottom of your pot with 1-2 tbsp of olive oil and cook them slowly over medium-low heat. You want the onions to become translucent and limp with absolutely NO browning. This is really important; if you find little browned bits, pick them out. If you're getting a lot of browned bits, turn down the heat. Browned bits will add bitterness to the sauce, and will also mess with the texture of the final product (no one wants crunchy tomato sauce!). This step can take up to 40 mins. Don't rush it.
Add the tomatoes (juice and all), the paste, the carrots, and about another 2 cans worth of water. [Note: You can use some red wine and some water if you want. This gives your sauce a much richer, deeper flavor. Sometimes if I'm making a double batch I'll make one with wine and one without.]
Now bring the sauce to a simmer over low heat and cook (uncovered) until it's reduced by about 1/3. This could take anywhere from 90 mins to a couple of hours. Don't rush it and don't raise the heat too much - you don't want to scorch the bottom.
You're almost done. Now you just want to add a bit of basic flavor to your sauce. Add in about 10-15 fresh basil leaves, finely chopped or cut chiffonade, about 1 tbsp of sugar (optional - I don't always put in the sugar), and some salt and pepper to taste.
Now for your final step, take an immersion blender and give the whole pot a few good pulses. How smooth you blend the sauce is entirely up to you. I like a few tomato chunks in mine, so I blend accordingly. (If you don't have a stick blender, you can move cups of the sauce to a food processor or blender, but you really should get an immersion blender if you're going to make this often - it's worth the $20 or so.)
Now you have a basic, no-frills, but totally delicious tomato sauce. Let this simmer for 30-45 minutes, giving it an occasional stir with a wooden spoon.
You can add veggies to it, add meat or sausage to it for a meat sauce, use it as a plain marinara, add some pepper flakes and spice it up, whatever you want.
I usually make 2 double batches at a time and freeze it for future use. I freeze it in Ziploc quart freezer bags (each bag holds about 4-5 cups of sauce). It's the perfect size for dinner for 2 - 3 people.