You cannot book dinner reservations before you arrive (you must have a room number to book). But don't worry, you won't have any trouble getting reservations. We don't like to be on a set schedule, and never know what we will be in the mood for, so we usually only book reservations a day or two in advance. Same with the spa.
Also, by waiting until you arrive, you'll be able to check out the daily and weekly resort schedule to make sure you don't schedule something over another activity that you want to do.