Lines have a general vibe (Carnival = party, for example), but I think the best way to pick a cruise is to find an itinerary you like. I'd target a specific cruise that it very far out, and decide on one that looks good, and then ask for that line. There are many different types of cruises, even within the same line. There are those that take you from one private beach to another, so that most of your time is spent either on a beach or doing beach/water activities. Then there are those that are basically a floating bus that allow you to wake up in a different city each day. And many in between. Maybe you want to experience the Panama Canal. Maybe you want to see Russia without the PITA visa process (only practicable way to do that is a cruise ship stop, but you do need a licenses tour guide/organization).
Where do you want to leave from and arrive to? Does it need to be the same place? Is there a cruise port near you, and if so, does it have itineraries that interest you? If not, you need to factor in travel, and likely a hotel for the night before, since flying the day of the cruise can create major issue (and expense) if you flight is delayed.
Also, you need to think about how much you can reasonably be gifted, and then how much you are willing to pitch in on your own. Unless you have very generous family and/or are wanting to do a 3-4 day Mexican party cruise, these can vet very expensive. With tips, fees, travel to and from the cruise, excursions, and alcohol if you drink, plus any other upgrades, the "ticket" price of the cruise can easily be 1/3 or less of the expense.
