Since this is your first cruise, I wholeheartedly agree with others that you really need to talk to a cruise specialist. Obviously, I'm prejudice on the matter, being one myself, but an agent can really help find the perfect cruise for you based on your requirements, lifestyle, and budget. They can discuss all the various cruise lines, ships, and ports of call. Whereas if you call the cruise line, you have to call each one and it's going to get confusing for you. Plus, if you book directly with them and have any problems, who do you think they represent? An agent is going to charge you the same as the cruise line and sometimes less, if they have any group space onboard, plus they may be offering some other amenities, like onboard credit. They charge nothing for their services, so it makes sense to utilize their expertise for free.
With that said, keep in mind that everyone has their opinion on what cruise line is the best. So remember; what one person loves, another will hate. It's all about finding you the perfect cruise for you, not finding the best cruise for someone else.
As for pricing, that is really going to vary by cruise line, itinerary, length of cruise, ship, and sail date. If you're flexible in your timetable, there are some great deals in September and again between Thanksgiving and Christmas.
But talk to your agent and they'll be able to make it alot less confusing and help you avoid all the hassles, while answering all your questions.
Let us know if what you're thinking as there are many people on this forum to help.