Not all cruise lines add tips and some you can prepay. The key is to look at the overall package and not just the initial headline price. Drinks packages are usually not worth it. Get an eSIM rather than buying onboard WiFi.
I personally book direct with the cruise lines but there are good specialist cruise travel agents. Cruise critic https://boards.cruisecritic.com/?&_fromLogin=1 can help you work out which line you might like.
Then use YouTube to look at specific cruise lines/ships. There are several popular channels that give you a good idea of the different options. The vibe on ships/lines can be very different and the size of ships can very enormously,
Fly cruises can be more expensive as flights add to the cost. Repositioning cruises, where a ship starts in one location e.g. the Caribbean and end in another e.g. UK/Europe, can be cheaper but may have a lot of sea days.
I always suggest a 7 day Fjord cruise at the end of May/ June is a good starter cruise. They usually have 4 port days and most of the time you are sailing in the flat water in Fjords rather than out at sea. It gives you a good idea of what you like/dislike and are not too expensive. The one thing you can’t guarantee is the weather.
Budget will be a big factor in determining which cruise line/cruise ship you select.
If you can give an idea of what type of holiday you like, I am sure posters can help you narrow it down.
I’ve done cruises with Marella, Royal Caribbean, Princess, Cunard and P&O.