Dining
"Wait, I have to plan my dining in advance?"
Kinda. Disney's dining options are generally split into two groups:
Quick Service (also called Counter Service), where you order at a counter, kiosk or mobile device, and then pick up your food and eat it wherever you can find space. Basically Disney's version of Fast Food.
Table Service, where you are taken to a table, a server takes your order, brings your food, and waits on you throughout the experience.
Table Service
For Table Service dining, you can make reservations (Advance Dining Reservations, or ADRs, in Disney parlance) for the duration of your stay 60 days prior to your arrival. You should take advantage of this opportunity, especially if you are wanting to eat at some of the more high demand locations, like Cinderella's Royal Table, Be Our Guest, or Chef Mickey's. Just walking up to restaurants and getting on a wait is a good way to waste a huge chunk of time just standing around, if they are able to accommodate you at all.
If you have difficulty getting a reservation you'd like, I can heartily recommend the TouringPlans Reservation Finder. You tell it where and when you'd like to go, and it will text/email you if something comes available that matches your criteria. You'll still need to go to the Disney site to actually make the reservation, but I've had amazing luck using this tool over the years.
Quick Service
If you're just planning on doing Quick Service, there's no planning that you need to do before you get to Disney, beyond perhaps selecting some places you'd like to try, if you care. You should, however, be aware that many of Disney's Quick Service restaurants utilize Mobile Ordering, where you can place an order ahead of time, and then pick up your order when you are ready for it. It saves a ton of time, and you should use it when it's available. You can place Mobile Orders via the MDE app, and breaks down as follows:
Place the order on the app.
When you get to the restaurant, click 'I'm here" in the app to let them know you're ready
Pick up your food at the designated Mobile Order counter.