It's highly recommended that you make ADR's (advanced dining reservations) for dinner, and WDW begins taking ADR's 6 months in advance. Some restaurants are very popular and are generally completely booked up weeks (maybe months) in advance (Le Cellier & MK restaurants come to mind). For lunch, you may be able to get in without an ADR to most places either right when they open or late afternoon.
I know it's a little daunting to have to plan where and when to eat months in advance, but you can always change your mind once you're down there. If you find someplace else you'd rather go, just call WDW dining or your hotel concierge and cancel your existing ADR (after making sure you can get into the new place, of course).
But by making the ADR's, you'll at least have a backup so you don't have to end up eating at times or places that are inconvenient or not to your liking. Have fun with the planning!