ok the traditional veg to use is: onion, garlic, peppers, aubergine, courgette, tomatoes, but you can add others if you like, I like button mushrooms in there too.
And because there's so much veg, it's quite handy to have two or three rings on the go at once or more, so you kind of cook the different veg individually or start them off separately at least and then put them into your biggest pan. You can do it in one pan but this way ensures things are nicely browned.
I tend to cook all the veg, then splosh in some wine/balsamic vinegar to lift the sticky stuff off the bottom and add flavour, and for the tomatoes it's good to use a tin at that point, seeing as it's hard to get juicy tomatos right now. Season, add tomato puree too if you want, use herbs like thyme or herbes de provence and turn down and simmer for a while, maybe half an hour, but not so long that the veg disintegrates.
Sorry, long - I love ratatouille!