Welcome to Cruise Crazies and the wonderful world of cruising. One word of warning - after your first cruise you will be hooked like all of us and looking to book cruise #2.
I think the most important thing is to decide how active a cruise you want and what ports you really want to see. The web has tons and tons of information to help you decide and you have a lot of friends here to help with information. Once you decide on a cruise line, set a realistic budget and begin to plan. The most important thing is to find a reliable TA and let them make suggestions to get you through the planning stages but don't let them talk you into anything you don't want.
Another important thing is the buy Insurance to cover your trip. Hopefully you will never need it and it will be a waste of your money, but should you need it for any reason it will be invaluable.
Enjoy planning your first cruise and as questions come up - ask away. We are here to help.