All the others have good ideas, and I'd recommend that you stick to one of the major lines for your first cruise. Carnival is going to be the most fun, for the money IMO. RCI is also great, but considerably more pricey for most any given cruise. NCL is good as well, but as another poster stated, I'd suggest that you stick to conventional dining instead of "free style" dining. You're going to enjoy the extra service you get when you have the same waiter, and bus boy each evening, as well as the others at your table (if you request a large table). Excursions can be a bit expensive, so do your home work on these, and make selections wisely. For first timers, I'd suggest that you stick to ship sponsored excursions, so you don't have to worry about missing the ship.
Whatever you select, I'm sure you'll have a GREAT time.