Tomatoes - this is the main ingredient...the base. How many you need depends on what kind you use and how much sauce you like. Personally, I think it's made best with the vine ripened kind, you know the kind that you buy still attached to the vine? I am not a fan of those big pumpkin looking tomatoes for Ratatouille. If you use a tomato the size of the vine ripened ones, you'll need about six.
Eggplant (about 1 or even half of 1)
Zucchini (about two)
Squash (2 or 3)
garlic (about 3 cloves minced)
and whatever herbs you like - parsley, oregano, basil, thyme
First, use your knife to make a shallow "X" on the bottom of each tomato. Bring a pot of water to a rapid boil and once it's boiling, drop the tomato in there for about one minute. You don't want to boil them, you just need some help getting the skins off of them. Pull them out after about a minute (you should see your X start to sort of peel back) and either put them in an ice bath or just lay them aside. Once they are cool, peal the skin off and set them in an empty pot. You'll want to slice them or pull them apart. You can then put in a little olive oil and put them on medium heat. Many people say to pull out the seeds before you cook them - because it makes the texture more smooth - you do what you like.
Then, cut your egg plant in slices, and salt them, lay them out flat or leave them in a colander for about 30 minutes to draw out the moisture. You want them to be more leathery when cooked, not mushy.
Next, get out your biggest saute pan and add olive oil, minced garlic, however much onion you want to use and saute until the onions are soft and translucent. Dump that mixture in with your tomatoes. Let that all sort of stew together.
Use the same saute pan and a little more olive oil to lightly saute your squash and zucchini and egg plant. And whatever vegetable you wanna throw in there to saute...don't saute it fully though, just make it a little tender.
Some people throw all of those veggies in with the tomato sauce and make a stew. I added a little bit of sugar and canned tomato sauce to my fresh cooked tomato - and sometimes I add diced tomatoes from a can if it's not soupy enough. After it's cooked a good bit and looks like real sauce, I put it in a casserole dish (I grease it first) and then I layer the veggies on top and then I bake it all for about 30 minutes or until the veggies on top all look tender. Then I put a little shredded parmesan on top and dig in. It's really good and actually easy to make, just a little time consuming. ;-)