Have a wonderful 1st trip!!! The things I think are probably most important:
1. When you say Christmas week, I assume you mean the week between Christmas and New Years-- If so, you need to realize up front that is Disney's busiest week of the entire year. It is going to be really crowded. You can still have a wonderful time- I've done it. Disney is beautiful at the holidays. You do need to at the very least have some sort of loose plan. You need to pick what parks you are going to what days- have the kids pick what rides and shows are most important as you are probably going to run into long lines. Be sure and understand and use the Fast Pass system. It is a huge timesaver.
2. If you don't already have a room booked, you need to do that NOW.
You already may have trouble getting what you want if you plan to stay on-property. As I said, that is probably the busiest week of the year. If you have trouble getting what you want, get something and then keep calling back and checking for cancellations, they happen. I've never stayed anywhere on Disney property that wasn't good.
3. As far as eating, you need to start trying to make ADRs (dining reservations) NOW, if you plan on having any table service sit down meals. We usually make 1 ADR a day, and do CS (counter service) for the rest. Allears.net has a list of all of the restaurants, where they are (which park or which resort) their menus, and prices. You can make dining reservations online. Dining reservations open 90 days ahead of time, so they are already open for your time period. You can make them online or by phone. You may have trouble getting what you want already. Book at least something, then keep trying for what you want. Cancellations happen. You will want to book a restaurant in or near the park you are going to be at for the day. It takes a long time to travel between locations at WDW. It is huge.
Have a wonderful time!