Welcome to Cruise Critic! As @Snaxmuppet and @Essiesmom stated above, your cabin will be assigned (probably) several days before sailing. The time you arrive at the terminal has no bearing on *that* part of your cruise. One thing that arriving early *does* do for you is give you a chance to have lunch in the Main Dining Room, starting your cruise off "in style". (We have NEVER gotten to the terminal early enough to do lunch in the MDR!)