I‘m a huge fan of cruises, been on a at least one a year every year since I was young.
It depends what you’re looking for. They will all have some form of bars, restaurants, entertainment, pools and kids clubs (unless adults only).
Princess is fab but can be quite pricy. P&O is quite British and has some really great facilities like the night nursery if you have small children. Celebrity is incredibly sparkly and modern and can feel quite a bit calmer then, for example, Royal Caribbean which is very fun and geared towards older kids with lots of activities etc. Norwegian was great as a bit of a party ship. I’m just about to go on my first MSC- I’ve heard it’s a bit more international, unless you go out of Southampton which they’ve geared towards British guests. Not that I’m sure what that means- I think there are kettles in the cabins!?
I would start figuring out where you want to go. As someone above mentioned P&O can be good for fly cruises as the flights are chartered and it’s such a straightforward process from check in to boarding. Other times the flights are separate and I would usually plan to land the day before the ship leaves, just in case.
One consideration for your 19 year old- if you’re on an American ship, in international waters, he very likely won’t be allowed to drink. This is as opposed to P&O where the drinking age is 18 as it’s a British ship. Just something to consider- we stuck fairly rigidly to P&O in the carribean when I was 18-21!
Drinks package on P&O is quite worth it, unless you like expensive cocktails and then you’re priced out quite quickly!
Personally I’m a fan of the new ships with the more modern design. Some of the old ships can be a lot of dark wood and carpet! You can usually Google any ship, and find out when it was either launched or refurbed. The cruise lines are also starting to come up with great tech, like the medallion class Princess ships which makes it easy to communicate and find each other.