What you might want to do is, since you know the month you plan to sail, go to one of the big travel sites (like expedia or travelocity) and select your month and see what's available. To narrow down your results I'd suggest searching by either destination or departure port (personally, if I can drive to the port I'm happier, but I know that's not an option for everyone!).
Once you've got search results down to double-digit options you can sort by price and eliminate anything that's way over what you can comfortably afford to spend. From what's left, list the lines and ships that are available and then go to the individual cruiseline's website and start reading about the ships available. Even though destinations are important, you're going to be spending most of your vacation on the ship so it should be the one you think you'll be the most comfortable with or enjoy exploring!
For instance, after my first cruise I was reading through the ship descriptions of the rest of Carnival's fleet and found that the Miracle is themed all in fictional characters (including some comic book references). As an avid reader and webcomic artist, I knew that regardless of where it was going, that was the next ship I was going to be on! Thankfully the port is within driving distance, the schedule worked out with my work deadlines and I hadn't been to any of the same ports on the last cruise.
Good luck and have fun