Just changed my coutndown calendar chalkboard thingy from 200 days to 199 - that felt GOOD :)
Hi Loletta! Your first trip - how exciting! I've never stayed onsite so can't advise about the dining plan, I'm afraid - head over to the Dibb where they know EVERYTHING :) I do know that they offer free dining plans sometimes so that would really be worth waiting for if the dates suited you as that would save you a ton of money.
You don't need a hrie car at all if you're only planning to do Disney parks. But you might want a car if you intend to visit other parks. There is some public transport but I'm not sure how easy it is - again, The Dibb.
Fastpasses I can answer :) They are only for Disney parks (not waterparks) and you can choose 3 a day. This is something that can be left until much nearer the time (you can't book until 60 days before you go) but it's quite easy - you set yourself up a My Disney Experience account and can book them on there. We can talk you through that nearer the time.
As for unmissable parks, you really need to have a read up about them to see what you personally think you'd like as it's a case of 'one man's meat'. For example, a lot of people rave about Animal Kingdom but I'd happily miss that one as find it quite boring. Likewise, a lot of people say that Hollywood Studios is the one to miss if you don't have time to do them all but that's a must-do for me! Briefly though:
Magic Kingdom - something for everyone. Not to be missed.
EPCOT - Probably more for older kids and adults? Split into 2 areas - one is a replica of different countries so prob boring for little ones (very few rides), other area has a few rides but not much for little ones. No thrill rides in the whole park.
Hollywood Studios - movie-themed rides and a few shows. Two of the best rides in Disney (Toy Story Midway Mania & Star Tours!)
Blizzard Beach & Typhoon Lagoon - water parks, something for all ages.
Animal Kingdom - safari park/zoo with a few rides.
Universal Studios & Islands of Adventure - Contrary to popular belief, something for everyone. Lots of thrill rides but also good play areas for little ones. The Harry Potter place!
Busch Gardens - animals and rollercoasters. Like Animal Kingdom but far more intense rides.
SeaWorld - does what it says on the tin :)