Hi, Jason!
Congratulations on your first visit to Walt Disney World! Be prepared to be amazed, too, Jason, as you've picked the most magical time of the year to arrive. :-)
Regarding your dining reservations for Table-Service Restaurants, I highly recommend that you do, in fact, make them asap. December is a popular month in WDW, and many of the most popular restaurants will actually book up as early as 180 days prior to Guests' arrival. During peak seasons (i.e. Christmas week or the 4th of July) I've seen signs in the parks stating that there are currently no Table-Service reservations available in that park for the day. While your early-December stay won't be as crowded as that, I'd hate for your magical first visit to be filled with disappointment as you spend a good amount of your time searching for a restaurant in which to eat or having to wait until 9 p.m. for dinner. That would be a sure-fire way to let a little magic leak out of your trip.
Instead, either go online to
Disney's online dining reservations and book a restaurant for each day that you'd like a Table-Service meal or call (407) WDW-DINE, where a Cast Member can assist you with reservations. I suggest that you consider your desired itinerary and pick a restaurant in the park of your choice (or nearby resort restaurant) and book those reservations quickly. Remember that you can always cancel those reservations, should your plans change, and this way, you'll be guaranteed that you'll have a place and a time to eat that's to your liking.
If you need help choosing or planning, simply send me another question; I'd love to help!