Hannah we're scheduled to dock at 8 a.m. (ship time) Local time will actually be 7 so you'll have have plently of time.
Hannah, You can book your excursion right away. My wife and I usually book online through local vendors and we've even booked excursions several times directly on the pier (trust me they'll be a lot of vendors selling excursions right on the spot)
The excursions on carnival.com are slightly more expensive than buying direct or at the pier, but for a first time cruiser I'd probably go through Carnival. There's a special peace of mind knowing that the vendors Carnival uses are first-rate, insured, and have a responsibilty to get you back to the ship safely. Then on your next cruise you can be a little more adventerous.
If you book through Carnival and decide to cancel or the ship is late getting to port they'll credit your sail and sign so no money lost.