Hi Josie:
It's never too early to plan a Disney World vacation. There are many great planning resources available to you, including the
official Disney Website, the
official Disney Parks blog, as well as unofficial websites and various books. Although always subject to change, as of now, dining reservations and character meals can be booked 180-days in advance. Note, if you are staying at a Disney-owned resort, you can make dining reservations for you entire trip (up to 10 days) on the first day of the 180-day window.
Summer can be a fairly busy time, so I would do your best to be ready to make your reservations 180-days before your trip. There are a number of online calendars that can help you count backwards from 180-days so you know when to start making your reservations.
Some numbers to keep handy:
407-WDW-DINE
407-WDW-STYLE (Bibbidi Bobbidi Boutique)
407-WDW-PLAY
Enjoy your hard-earned vacation.