Welcome aboard, Teri! Congratulations on booking your magical sailing with Disney! Choosing a Guarantee Stateroom is a nice way to lock in a price and know you'll get at least that stateroom level, but possibly be put into a higher level stateroom. I've had great experiences with Guarantee staterooms. We found out our stateroom assignment a week or two before sailing, though it can vary. Either way, you'll have your information when you arrive at the ship and you'll be all set.
Online check-in begins 75 days prior to sailing for
first time cruisers with Disney, and 90 or more days prior for guests who have sailed with Disney before (Castaway Club members). It sounds like you must be a Silver Castaway Club member if your online check-in begins 90 days prior. That's great news as you'll have earlier access to some of the special events and extras that are able to be pre-booked. Things like Port Adventures,
Adult Exclusive Dining reservations, and some ticket-only events. These are
character events that require tickets, some carry additional cost, like the Royal Court Royal Tea, and others are complimentary, however they do require tickets. They're popular, so logging in as soon as you're able allows you to book the events your family really wants before they're sold out. If you think you'd like to attend the Frozen Meet and Greet or the Princess Gathering, among others, you'll surely want to reserve a spot as soon as you're able. Some Port Adventures book quickly, as do Bibbidi Bobbidi Boutique Reservations and Palo and Remy's best reservation times go quickly.
I hope that knowing that ahead of time allows you time to decide on any must-do's so you're ready to log in as soon as you're able to begin booking! Have fun planning, Teri!
Marcy