This is why it's so very important to work with a good travel agent, especially one who is a Cruise Specialist. They can answer all your questions and help you to find the right cruise for you based on your needs, lifestyle, and budget. You can spend hours and hours researching, but a good Cruise Specialist can make it alot easier for you. Plus, their services are free, so it makes sense to utilize their expertise and knowledge.
Keep in mind that everyone has an opinion and what one person likes, another will dislike. So stay away from 'recommendations'. A good Cruise Specialist will provide as much information as you need to make a decision based on what's right for you and not what's right for them or someone else.
The information that Paul has provided is a wonderful starting point, but you'll have many many questions and you need someone who will take the time to answer all of them. And do NOT book it through the cruise line or one of these big online sites. Whenever you call them, you're dealing with someone who is not a travel agent, works in a call center, has been there less than six months, and has never even seen a ship or been to any of the ports. Their information is extremely limited and they're only interested in making a sale and not in providing service. A Cruise Specialist will make sure you get the best price and has your best interest at heart. Plus, if you have any problems, they represent you and have contacts not available to passengers.
Sorry, didn't mean for that to sound like a sales pitch, but having done this for so long, I really hate to see new cruisers get caught up with all the hype you find on the internet while being confused with all the information.