Hi Maria:
To help you decide the best order to visit the parks, I would check out the various park calendars for park hours:
Magic Kingdom,
Epcot,
Hollywood Studios, and
Animal Kingdom. The December park hours are not yet on the website, but you can expect them in the coming months. Park hours are important in making your decision on "park order" because if you are staying on property you can take advantage of
Extra Magic Hours (or extended park hours), which offers Disney World Resort Guests the ability to enjoy additional time in Disney Theme Parks. Typically, one of the 4 Walt Disney World Theme Parks opens one hour early or remains open up to 3 hours after regular Park closing time.
That being said, if you want to concentrate on one particular park, I would try to schedule Magic Kingdom early in your visit. There is so much to do at Magic Kingdom that you may want to commit a second day to the park.
I also like your idea of going to visit the restaurants, shops, and attractions in
Downtown Disney. I would carve out at least an afternoon or evening to spend there. If you are staying on property, you can take Disney transportation to and from Downtown Disney to and from your resort.
Since it's your first vacation, I would visit Disney's In-Depth website on "
Planning Your First Trip"
Enjoy your vacation.