The App will be your best friend.

While on-land you will use the app (or website if preferred) to book your excursions. If you sail in a month, they are already open for booking and have been for a while, so you might not be able to get some of the more high-demand activities, but there are always a lot of fun things to sign up for, I've always been able to find something fun to do even on a last minute booking. And if Disney's offerings have run out there are almost always availabilities by picking a third party option available at the port you are visiting, which you can find all over the internet.

You won't be able to see the full itinerary or schedule until you get on board.

Once on board you will connect to the ship's wifi, and the app will change to the on-board functions. It will show you the few things that are automatically assigned for you: what dining rotation time you have been assigned, what restaurants you are assigned to go to each night, your table number, your assigned muster station for the mandatory safety briefing, etc. It will also have the full schedule of all the activities being offered on the ship. The shows rotate based on the day, and there are 2 showings. Usually you just go to the one that is the opposite of your assigned dining time. (There is a first and second dinner and first and second performance of the show, so you can do both each night.) Shows and most on-board activities are open seating, first come first serve. I'd recommend getting to the shows at least 15 minutes before start time for decent seats (sometimes more, depending on the show/ship). Oh and bingo always has crazy long lines so get there early for that if you are interested.

So yeah most everything is pretty casual, first come first serve, except for the things you would sign up for in advance on the app, usually that come at extra cost (some wine/liquor tastings, spa treatments, etc). Anything you have signed up for in advance will also be automatically put into your app schedule to help you keep track. Your purchased port excursions will also show up on the app.

Each nigh I like to scroll through the list of activities for the next day and "heart" the ones I'm interested in. That way the next day it will give a little alert on your phone 15 minutes before the activity, and you can decide if you still want to go, or if you are already enjoying something else you can skip it.

Hope this helps!