Don't be nervous! The Oasis is one of the most stable ships ever built. You usually won't even know you are moving. All modern ships have stabilizers which are like the fins on a humpback whale. They minimize roll, but as I said the Oasis has a very stable design and you wouldn't feel any motion unless it was really rough - and it usually isn't.
Excursions you can book on-line will close out soon for a March 26 sailing, so you likely will have to wait until aboard the ship to make any reservations for tours.
Nobody can really recommend tours for you since it depends on what you like or want to do. But then the Oasis Of The Seas is it's own excursion, with so much to do and see. For a first cruise you are starting out at the top as far as going on an innovative ship with tons of choices for virtually everything.