They normally open up reservations the day before for the specialy restaurants. They always have a half off offer the first night of the cruise, so book Le Bistro just to get things going. The best way to do it is to board with your restaurant choices already made, that way you can call the instant they start taking reservations and pretty much get whatever time you want.
The first day at sea will be an optional formal night, but I would still suggest dining in an alternative restaurant, as they are so good. They are worth the extra cost.