Welcome to the cruisers waiting room, quite the happy place.I envy you the planning of your first cruise, as to me, research is a fun part of it.
Since it appears you are in the UK, I am going to toss this out...For your first cruise, why not do a Med itinerary. Most from across the pond, when they sail from Florida,with the cost of airfare, take longer cruises. Since you are so close to departure ports in Europe you can save on airfare,and get a longer cruise, almost in your back yard.
A cruise specialist can walk you through the steps, to make sure you select the right ship and itinerary, for an exciting adventure. Getting the right cabin,and placement of the cabin, too can enhance your experience.
I sailed the Carnival Magic in the Med in May,and it was a glorious time of year, to sail. These crusies are port intensive, and the excursions can last all day, so, your ship time will be less, then if you went with a Caribbean cruise.
If indeed it is the heat and palm trees of the Caribbean, you seek, consider that in the summer, the ships are filled with kids. Some areas of the USA, start back in August, with the balance right around Labor day.
Another inportant factor to consider is getting travel insurance..a very important part of feeling comfortable it by chance, you find you need it. We did, soo I speak form first hand experience....so, did we help you a bit? I hope so...come back and tell us hpw you made out..