Seeing it is your first cruise you will find many things you want to do. Try not to do everything and focus on a few things that you can really enjoy. Look on the Carnival web sites to read what tours they have even if you do not want to take those tours you will get a description of tours. Once you have the description you can ask on the boards what independent tours are similar. If you see tours you want to take through Carnival book them right away once onboard the ship.
Your first day onboard can be confusing so I suggest after you have checked in to take a floor plan map and start walking each deck and make not of the places you think are to your liking so you can find them a little easier later on.
Most of all let the crew do their job and relax and have a great time.