Welcome Iowa Girl,
As for dates, just go to disneycruise.com and go either to "make a reservation", or "rates & dates". That will give you the specific cruises available and sailing dates for each. I'd also suggest you order the cruise video while on disneycruise.com.
We booked through All Seasons Travel (allseastravel.com) and at the time of inquiry they had by far the best prices. They will rebate you if the cruise for some unlikely reason should ever go down in price. Earlier you book the less money.
As for rental cars, I don't know where you could turn the car back in at Port Canaveral. There may be someplace I haven't noticed. Also, I've read other posters discuss one person driving the rental vehicle back to MCO and taking a shuttle back alone. I think this was families that drove down, rather than flying however.
Glad to see you're excited as a first time cruiser - you should be!
Bill