Tomato sauce depends on what you're making it for. If you're going for pizza sauce, you don't want loads of add-ins like onions and mushrooms and (pleh) green peppers. It also depends on whether you have real live tomatoes or are using a can of crushed tomatoes. But my basic process is always the same.
What you need:
can of tomato stuff, or, tomatoes. if I'm just making pasta for myself, I use about four squash-ball sized tomatoes.
lots of garlic (3-4 cloves per can)
other sauce veggies (I'll use half a big onion or a whole small onion per can, cut into wedges)
italian seasoning, or basil and oregano
salt, pepper, and chili flakes
a note: if using real tomatos, I toss them in boiling water for a minute so their skins split, and then let them cool for a few minutes and peel them, the skins come right off. when initially cooking, the sauce will be fairly watery. it takes around 15 min to reduce to a nice saucy consistency.
Heat your saucepan and put a little olive oil in. Chop up your onions, and throw them in the pan. Mince the garlic, and throw that in too. Sprinkle some chili flakes in. Once these things are mostly cooked (you don't want the garlic to burn or go too brown, but you want onions pretty soft), throw in tomatoes (or canned stuff). let this sit for a while (five minutes at bare minimum), mashing up the tomatoes to help them cook. add loads of italian seasoning or basil and oregano, some salt, and lots of pepper. if it's very acidic, add a pinch of sugar. cook for at least another five minutes. taste, add more seasoning to taste, and let sit a little longer. The herbs take a little while to properly flavor the sauce, so tasting right after you add won't give you a proper idea of what it will taste like.
Kathleen, if adding a pinch of sugar didn't help, I suspect you aren't putting in enough seasoning. If I'm cooking a whole can's worth of sauce I probably use on the order of a tablespoon of seasoning all together. I start cooking the sauce before I start cooking the pasta, and finish after the pasta is cooked. It should take bout 20-25 min from start to finish.
For pizza sauce, I don't use onions. So I toss in my garlic and chili flakes (lots of them, since it will get spread out over an entire pizza) and when the garlic is just starting to smell delicious, I throw in my tomatoes. It takes at least 20 min for the tomatoes to reduce to a proper pizza sauce consistency. I don't add nearly as much seasoning, just a bare amount of oregano, and some salt and pepper and slivered fresh basil.