A cruise is my idea of hell, I do not like being on water and the thought of trying to sleep on a ship, just no.
However I am fascinated by all the videos on YouTube so hear is what I have learned from them, and bear in mind I have no direct experience.
1 you need cruise insurance, not just travel insurance.
Decide what type of ship and line. Some are more kid friendly, others are adult only most are somewhere in between.
You didn't mention if you have children but if you do have a look at what is available for them.
Decide the size of ship, some are virtually cities with entertainment on a par with the west end. As someone said theses ships can't always get as close to the port as smaller ships.
Smaller ships tend to have more personal service and can get into smaller ports.
Nationality / language of staff and fellow passengers. On Costa cruise all the announcements were in 7 languages which takes time. If you don't speak another language and you are sociable then pick a ship with mostly British and other English speakers. If you are on a ship with mostly southern European passengers then expect children to be up much later than British children normally are.
Do you want to walk off the ship at ports or are you OK with 'tender' ports where you are ferried from the ship to the port in the life boats.
How many 'sea days' do they have? As Gary Bainbridge (YouTuber) says an atlantic crossing with 7 sea days is not the place to learn you do not like sea days.
Ships can miss ports, due to bad weather or other reasons. Your cruise insurance will usually make a payment if they do. They can also substitute ports so you may miss Rome but visit Sardinia instead.
Different ships have different dress codes. Cunard seem to have the most formal nights and the best afternoon tea.
You can usually wear what you want in the buffet restaurants.
Do you need a kettle? British ships will normally have them but not others, and no you can't take your own on board, electrical items are restricted due to fire risk.
You cannot take a lot of alcohol on board, one or two bottles of wine.
Excursions. The cruise want you to buy excursions from them but often you can walk a few meters and book a cheaper one from a local company. This sounds like a no brainer BUT if the excursion is late returning the ship will wait for you if you have booked through them, otherwise it won't.