All cruises are not created equal. They are not even equal within a specific cruise line. If you are not a seasoned cruiser I have a few things you may want to consider as you choose your next cruise.
First, is age important to you? Longer cruises and those farther away than the Caribbean will attract the over fifty crowd more likely than not. They have the time and the money for those longer more expensive cruises and can go at a shoulder season (the months just before and after summer vacations). Younger folks will take shorter cruises and more likely cruise during the summer months when kids are on vacation from school
Speaking of kids, are you planning to travel with them? Some cruise lines are a little more attractive to kids than others. Of course Disney is the obvious but that can get a bit pricey. Here's where a good travel agent can help you out. A good TA should be able to steer you to a line that is catering more to kids and has a really good program for them. A good kids' program gives you some free time to enjoy and relax as you cruise.
This year we did something a bit spontaneous and booked a four day cruise in the Caribbean. We did so knowing that this retired couple would likely be mixing with a ship full of twenty-somethings since it was spring break time. Sure enough there was a party going on at the pool 24/7, or 24/4 as the case was. We weren't interested in hanging out at the pool anyway and enjoyed our cruise mostly because we expected there would be scores of kids and energy and yes, drinking. Had we gone with the expectation of a quiet cruise ship (not that there weren't quiet areas we found) we would have been disappointed.
Again, a good TA will help you with timing, destination and the type of cruise line that will suit your expectations. Start your cruising experience by booking through an agent. Then if you want to explore options on your own, you'll have some experience and a better idea of what you like.