Take the easy way... buy one called "Italian Seasonings". Just about any brand -- they are mostly the same. An average size pot would take 1 to 2 tbsp of it, depending on how much you're making.
Although it is already in the Italian Seasonings, I usually add some extra Oregano too. About half as much. Then be sure to add some fresh garlic (although garlic powder works well, too). Definitely add some chopped onion (half of one to a whole one depending on how much sauce you're making), and half to a whole green pepper, chopped finely. The bay leaf or two mentioned earlier, is also a really good addition.
I love making spaghetti for my family. To get the right taste you have to add spices. I like to use 2 bay leaves, one teaspoon of salt, one teaspoon of oregano, 2 teaspoons of dried basil, and half teaspoon of black pepper.
The husband likes his spaghetti sauce on the spicy side. I'll throw in a teaspoon of red pepper flakes for him, when he's been good :) My personal favorite: fresh basil, added at the end, on top of your plate. Use a good knife, so as not to 'bruise' the leaves. If you like olives like we do, toss a handful of PITTED olives in the pot, and some of the juice, straight out of the jar, too! Yum.