Welcome, Angela!!
I completely know where you are coming from when you say you are overwhelmed. There is a ton of information out there. And the more you know, the better your trip will be. Luckily, you have come to the right place. People here are very friendly and helpful. Trust me when I say that as hard as it is to believe, since you have a year to plan there will probably be a point in time where you have planned everything you can and will be just waiting for a time when you can plan some more or just wait for time to head to WDW. That time is awful, even worse than the overwhelmed feeling that you have now.
You've already had some great suggestions. Get yourself a few planning guides from the library. Birnbaums has great photos for the kids to enjoy. Unofficial Guide is packed with great info. PassPorter is a wonderful organization book as well as a guide book (My personal favorite, too.). There are really all kinds of books out there on WDW, but if you check out the ones at the library you will know that you are getting the one that you prefer.
Another great suggestion for planning is to break it down into small chunks. You've already made one of the toughest decisions which is WHEN to go. I've never been in December, personally (we are more value season type people), but I've heard that the parks are gorgeous then. The topics that I generally break our trips down into are:
When to go?--You already have this one down. Things that can play into this is work and school schedules, crowd levels, weather, cost, occasionally other things going on in the parks, ie. Festivals, holidays, special events, etc.
How to get there?--Transportation might help you finalize what your travel dates are exactly. With so many family members, you will have to decide if you are driving, flying, traveling by train, etc. If you decide to fly, sometimes you can get discounts by flying on off days. Once you know exactly when you are going to arrive, you can set up hotel reservations. Another thing you might need to consider is whether or not you are going to rent a car. Mousesavers.com has some great info about renting a car that includes discount codes for almost all car rental companies. Incidentally, this is a great all around site for saving money!
Where to stay?--- On site or off site? This is the part of planning that I like the best, and I confess that I change my mind many times before finally settling on a place. We, too, have 5 kids although ours are older than yours. We have stayed both on site and off, and for us off site works best. I know that it isn't for everyone, but we enjoy the extra space and can even save money by renting a 4 bedroom vacation room with private pool, renting a car, and paying parking vs renting 2 rooms at a value resort. As I said, it isn't for everyone and many claim that they would miss out on the 'magic' leaving Disney properties. Staying on property also gives the benefits of using WDW transportation including Magical Express, so you theoretically never have to drive yourself anywhere. Another perk only available to Disney World Resort Guests is the
Disney Dining Plan....but that moves you to the another planning section---Food.
Where/what to eat?----For me, this is the toughest planning section of all. Lots of decisions are going to be needed here. Do you want character meals? (I recommend at least one seeing the ages of your kids!) How many sit down meals do you want to do each day? What types of foods do you like? Either guide book, Unofficial Guide or PassPorter, will help a lot with a general overview of each restaurant. Allearsnet.com has a great section with menus from each of the restaurants to help you fine tune your decisions as well. Once you have an idea of where you want to eat, you can make a more educated decision if the Disney Dining Plan is right for your family. (It probably will not be worth it if your family prefers to eat mostly counter service meals, for instance.) I'll talk about ADR's in a minute.
Which day, which parks?--- This can be a pretty important part of planning, as well. You want to try to avoid crowds, if possible. They can really put a crimp in your enjoyment. There are two web sites that run services that you can pay for which will tell you when to go to what park. They are TourGuideMike.com and TouringPlans.com. Both of them are reportedly very accurate. I'm also told that you can figure this out on your own just from reading these boards. You have plenty of time to do the reading, but if you feel that they would be advantageous to you, by all means check them out. Once you have your plans set and you are within 180 days of your trip, it is time to make your ADRs.
ADRs----(You've got LOTS of time to work this out.) You need to call in Advance Dining Reservations (ADRs) for the table service restaurants as soon as you can. It is becoming more and more important as the table service restaurants are booking up more and more quickly. This works out to be 180 days before you will be at the restaurant...unless you are staying on site. This is a little confusing, but stick with me. If you are staying at a WDW resort, you can make the ADRs for your restaurants for 180 days from your check in PLUS an additional 10 days early, which basically means that for most people you can make all your ADRs with one call instead of calling back each day to get the 180 day time.
I know that this is probably a lot to throw at you, but once you start your planning you may find that you really enjoy it.
Feel free to ask lots of questions. You'll have a bunch as you go along. People here will be very glad to help you!