I will try to give you my own personal take. First of all, welcome aboard! I never thought I would like a cruise, but now I am hooked. I would advise that you sail from Galveston for your first one. Being able to drive to the port is a big cost advantage. There is a travel agent at galeveston cruises .com that I have used both for information and to book with. At their website you can see the price and selection of ships/itineraries from Galveston. The time of year is hurricane season and therefore you may get rough seas, but the price is usually better as well.
Destinations and cruiselines are a matter of personal choice, but let me say this. You do not have to be a party animal to enjoy carnival. You might check out the Conquest for a good itinerary from Galveston.
Let us know where you decide to go, have a great time!