Hi Jodie,
Welcome to the Disney Parks Moms Panel! How very exciting that you're making plans for your first Walt Disney World vacation! You have so many magical memories just waiting for you.
In most cases, you can book a vacation directly with Disney without including a dining plan and then make that addition later. I actually do this quite often myself since I like to be sure I have my Disney Resort hotel and dates confirmed just as soon as possible, but I'm not always ready to decide on the rest of my vacation details. Just keep in mind that to add a dining plan, you must book a vacation package that also includes a Disney Resort hotel and theme park tickets. The current rule is that you must call at least 48 hours before your arrival date to convert your reservation to a vacation package, so just keep that in mind when you're making your plans. To make a new reservation or to make changes to an existing reservation, you can call (407) W-DISNEY (934-7639) and speak with a Cast Member.
If you're booking with a travel agent or a third-party site (such as Expedia), be sure to check in advance if this is possible, as those rules can vary quite a bit.
I hope that helps, Jodie, but write me back if you have any other questions as you make your vacation plans.
Wishing you a magical vacation and delicious dining!
Beth