Along with the links that slapshot provided above. For some of the table service restaurants (I've done it for Magellan's and Blue Bayou) you can make a reservation the same day in the park. Just go to the restaurant and ask to make a reservation. This is only if they have slots open. Depending on how busy it is you may be able to select your preferred time, or they will try to give you the time closest to what you'd like.