If you're not picky about where you go, ask of course what deals there are. Don't forget to account for airfare to/from the port, transfers from the airport to the port, and tips for crew members. A good travel agent should sell you a package that covers all that. Also check with orbitz, etc.
Most things on the cruise are free, such as entertainment, food and non alcoholic drinks. Booze, gifts, and anything from the ships store is not included. Any shopping and excursions in the ports are also not included. You will need a credit card to give the cruise line so you can get a ship card, because they are pretty much all cashless. However, you'll want cash for any shopping in the ports.
You may need a passport, and those are taking a while, so get on the ball and get that done now.
Have fun!