Booking a cruise is not the time to be your own travel agent. Anyone new to cruising should use a cruise-specialist agent. They help find the best cruise line to meet your tastes and choose the right cabin from among the different grades and types.
You often get better deals using an agent than going direct as they may offer extra discounts or benefits, like on-board credit. Cruise lines are different and many first-time cruisers do not enjoy their first trip by choosing the wrong cruise line.
For example if you like dressing up, then Cunard has more formal nights and is strict about people sticking to dress codes, even informal nights requires men to wear jackets. If you dislike having children around you then P&O offers some adult-only ships. Carnival is informal and fun focused with lots of activities on deck and so ideal for people that like a party. Agents have the knowledge to navigate you through all the choices.
Cruise fares are complex and ships have dozens of cabin grades and permutations. You need an agent to help you through these to ensure you get what you need.