To get the best value, think timing. First when you book and second when you sail.
Booking: If you book around a year in advance, many cruise lines will give you a substantial early-booking discount. By booking well in advance, you also get the best choice of cabins. I would recommend any good, cruise-only, full-service travel agent to help you if you can book early.
The real bargains are found 90 days or less before sailing. But there are fewer choices available by then. My favorite agent for last-minute cruises is www.vacationstogo.com They will be happy to help you by phone. I don't know whether they provide enough service for a first time cruiser.
Sailing: The lowest fares are usually late August through the fall. Stay away from school holidays, especially the week between Christmas and New Years.