You select first or second dining seating when you first book a cruise. The default is second seating, so if you prefer first seating, make sure you ask for it (or have your
travel agent ask for it).
Almost everything that can be booked will be available when your booking window opens 75 days before your cruise. In addition to what lklgoodman listed, these include the Bibbidi Bobbidi Boutique and
Castaway Cay Cabanas (although the cabanas are very likely to be fully booked before the 75 day mark).
As of now, there are a small number of character meets that can't be booked until 30 days before the cruise. Most character meets don't require registration at all -- you just see characters and join a line if there is one. The princess gathering, Frozen gathering, and some Marvel, Pixar, and Star Wars meets do require free booking in advance, as do character breakfasts when offered. I think the Princess Tea, which does have an additional cost, is also booking at 30 days.
However, you don't "need" to book anything. There will plenty to do on the cruise that doesn't require advance planning. When your booking window opens, you can look at the port adventures, dining, and onboard activities that are available each day. If what you wanted isn't available, you can keep checking online, in case there are cancellations, and you can also check once you are onboard.