One of the nice things about booking ship's excursions is that they take very good care of you.
As others have noted, you will usually find your excursion tickets in your cabin when you board. The tickets will specify the meeting place and time, and frequently have other notes like "bring sunscreen" or "wear comfortable shoes."
Then you will pretty much be escorted through the whole thing. Either you will be escorted from the meeting lounge, or you will be met by shore excursion staff as you leave the ship, and they will see that you get to the right place. Having the shore excursion staff to look out for you is very comforting, especially for first time cruisers.
The other nice thing is that in case of any sort of delay, caused by late arrival or weather or whatever, once again the excursion staff is monitoring it. Announcements will be made throughout the ship if it is necessary to change any excursion times.
On most lines, even if you prebook excursions, they are not charged to you until you get on board and then they are put on your shipboard account. You do need to note cancellation procedures and penalties. Most of the time you can cancel without penalty up to a few days before the excursion but this is not a universal rule. Check it out.
I know you will have a GREAT cruise!