Welcome to planDisney, Keri. We're delighted to have you here. 
Dining plans can be subsequently added to a Disney Resort hotel-inclusive packages booked directly with the Walt Disney World Travel Company. If booking with a third-party vendor, my understanding is that you can add a Dining Plan at the time of booking.
You can choose between the 
Disney Quick Service Dining Plan and the 
Disney Dining Plan. The latter includes Table-Service meals, including Character Dining, and is considered the fancier option of the two. 
The choice of plan depends on what works best for your Guest party. For us, the flexibility of Disney Quick Service Dining works for our young children and appetites.
To add extra fun to your dining experiences, try some 
Character Dining! If this or other Table Service dining options are of interest, I recommend making reservations 60 days in advance. Popular locations, like 
Story Book Dining at Artist Point with Snow White, tend to fill up quickly.
A fun and insightful 
planDisney podcast details everything you need to know if you need more information before deciding upon including a Disney Dining Plan! 
Keri, I hope this information helps today. I know that many magical dining experiences await you at Walt Disney World! Have the best time!
Louise