I would start by deciding if you will be doing the dining plan. That way you know how many counter service and table service meals you need to plan. Are you interested in character dining? Are you all big breakfast eaters? Are you more adventerous or very simple in your tastes? You can look at menus at allears.net. They have them broken down by parks and hotels.
Once you decide where you want to eat using what the other posters have suggested, you call the dining number: 407-939-3463 (it has the word DINE in it but I have spelled it out).
What I do is have the dates/suggested time and restaurant on a piece of paper in front of me before I call. For each restaurant the CM will ask suggested time ... etc ... but it is important to RECORD the confirmation number. Do not LOSE the confirmation number. Although I have never had difficulty sometimes having that clears up mysteriously lost ADRs when you are onsite!
Now, the window for reservations due to the new system is odd right now. If you are making reservations at Chef Mickey or Victoria and Albert or Cinderella's Royal Table it is definately 180 days before arrival (if staying onsite). Others (unless you are arriving before Feb 22, 2009) must wait until 90 days before (or some such thing -- I don't know - it keeps changing!)
Anyway - it doesn't really matter - if you call and it is too early -- they will just tell you when you need to call back.
Have fun - I LOVE deciding where we will eat and perusing the menus! Don't forget to look at Counter Service Menus too - not that you need ADRs there but it is good to get an idea where you might like to stop for a quick meal!
Once you decide where you want to eat using what the other posters have suggested, you call the dining number: 407-939-3463 (it has the word DINE in it but I have spelled it out).