I'd start with what are your must haves, must not haves, can tolerate.
How do you feel about children? You would not avoid them on Disney but other lines are adult only.
Do you like getting dressed up for dinner or prefer to be in jeans / shorts at all times.
What sort of things do you want to experience? A Caribbean cruise is lots of sea, sun, beaches, a river cruise through Europe gives you more chance to experience culture. Alaska is another thing altogether.
What sort of entertainment do you want to see in the evening?
What sort of things do you want to do in the day Quizzes, lectures or zumba? Go carts, roller coaster or gym?
How many sea days do you want?
Are you bothered by the languages spoken? You will find people who speak English on all cruises but on something like Costa you might be in the minority if you don't speak Italian.
What sort of cabin do you want? Are you OK in a room with no windows? Do you want a balcony?
Is there anything you would be devastated to miss? Cruise ships sail where the calm water is so sometimes miss ports or change them.
I would hate to go on a cruise but I'm fainted by a couple of YouTubeers who do cruise and film their experiences.
If you are wanting to cut down the expense you consider a 'repositioning cruise'. The same ships that cruise around the Caribbean in winter are the same ones that cruise about the med in Summer, obviously the ships have to get from one 'home' port to another. They have multiple days at sea as they cross the Atlantic.