How do you reserve in advance for that or can you reserve when you get on the boat????

you can make advance reservations from your mycruise section on carnival.com  or just wait until to get onboard

We have always waited until we get on the ship and you can call directly from your cabin to make the reservation.  We have never had a problem getting the day/time we wanted.  And if you do change your mind after you make your reservation there is no problem cancelling it.  LMP

One thing that I want to keep in mind prior to possibly booking the steakhouse is not to coincide with the formal night.  I think formal night may be the night they serve lobster and I wouldn't want to substitute the steakhouse for lobster.  I guess I should check out the menu first....that way I can take it all in.