Ok, here's the scoop. I go on cruises every year and am up to about 15 now.
You will find the most affordable cruises are done by cruise lines like Carnival, Royal Caribbean, Norwegian, Princess, Celebrity, Holland America, and Disney. Most of the ships for these lines are now 1500 to 2500 passenger ships. The ships are big but things are so well managed that you really don't feel crowded on them. Prices on these ship can be as low as $800 for a 7 day inside cabin and about $1200 for a balcony cabin.
The first rule in getting less expensive cruises is book early. The second rule is go with the same cruise line multiple times because they do offer repeat customer discounts of $50 to $100 per person in a cabin. The third rule is shop around for specials. Some lines will offer specials on a date that does not usually sell out, like Christmas or Thanksgiving week. So don't lock yourself in to one particular week of the year; be flexible.
If you have not cruised before and don't know the ropes it may be beneficial for you to use an agent; the agent does not charge a fee but is paid by the cruise line. But, also, do some home work on cruise line, itineraries, and ships. Use the cruise line web sites to look at what's being offered. I will list some below. Try to have in mind some of what you want to do when you talk to an agent. If you feel comfortable doing it, you can do your own booking online, the price is the same and you can do everything you need right on the web sites. The sites usually have cruises listed for this year and next. I booked my last two cruises completely online, including making payments and booking port tours. The format and service on all of these is about the same.
I have found that the food on Celebrity, Princess and RC is a cut above what I have been told is served on Disney and Carnival. Carnival is the fun ship, but their accommodations are not the best. Carnival is the party cruise line that most young folks and first time cruisers take (ages 20 to 35) who want to party, party, party. If that’s what you are looking for then Carnival is the one for you. On the other hand, Princess, Royal Caribbean, Holland America, Celebrity, and Norwegian tend to have people who have cruised many times and are in the 40 and over age group; a more reserved crowd.
Sometimes people get confused by the categories on ship that agents or the web sites refer to. Categories are simply a listing of the different types of cabins on the ship by size, location and amenities. Categories are the price structure. What you need to know and understand is that the category designations DO NOT mean the same thing on all cruise lines. the terms "cabin" and "stateroom" mean the same thing; the cruise industry prefers stateroom. The standard cabin that most people get (inside/outside/balcony) are generally between 175 and 200 Square feet, about the size of a 9 by 12 ft home bedroom (balconies are about 45 to 55 sq ft). Each cabin has a two beds which can be made into a queen, a bathroom with a shower stall, a closet and some drawer space, a TV and telephone, a safe for valuables, and a life jacket for each passenger.
Once you are on ship the staff caters to you, its whatever you want. There is something to do all day every day. When you arrive at the port to check in you (each passenger) will receive a key card which will serve as your room key, charge card for anything you buy on ship (i.e.,drinks, souvenir) and its also your pass to get on and off ship in each port. When you first board the ship the security personnel will have you insert your key card into a scanner and then will take a photo of you that is included in the magnetic strip on the card. Each time you get off ship you will need this key card and many times also a photo ID (passport or driver's license). This card allows them to keep an accurate track of all passengers entering and/or leaving the ship. Each day of your cruise you will receive in your cabin a "newsletter" listing the time, place and particulars of everything happening for you on ship. There are generally activities like demonstrations of ice carvings, napkin folding and towel folding, and cooking demonstrations. Some ships may offer a galley (kitchen) tour or a tour of the bridge. There may be organized games and other activities around the pools. In the evenings there are generally shows, one show for those with an early dinner seating and another (same show) for those who eat late. There is always food available. Usually there is a place in ship where you can go get something. Or, you can always just pick up the phone in your cabin and order from the room service menu (food is free). If you want, you can order breakfast room service.
All ships have a casino with slots and table games. Ships have movie theaters, generally small but nice, and you can also see movies in your room. They have ATM’s and a purser’s desk where you can cash a check or exchange currency if you need to. There is a tour and activities desk so you can schedule land tours and activities. Ships have free morning exercise programs, stretching and walking, etc, and on some you can pay for other things like Spinning. All cruise ships have gyms so you can go workout, usually any time from 6:00 AM to late at night. There is usually a walking/running track on one of the decks also. Some ships leave the gym open all night.
There are always pools. Some ships have more than one and the newer ships generally have an indoor pool as well as outdoor ones and all of these have hot tubs. There is generally a basketball court on the top deck, a place for scuffle board, and a mini-golf course. The newer Royal Caribbean ships have a rock climbing wall and the newest one has a pool for water surfing.
There are places on ships to play cards and board games, and the cards and games are provided. If you want to just sit and read, there’s a library where you can bring your books or select one from the shelf. Ships have internet cafes and some offer WIFI if you bring your prefer to use your computer in your cabin (some ships offer notebooks for rent). There are cocktail lounges that feature a variety of music. Some have a piano bar with jazz. There is usually a disco that starts after 10:00 PM each night and last until everyone leave. On certain days ships will have special activities like a chocolate lovers buffet, or a midnight deck party .
I do recommend getting a balcony cabin. We had a balcony cabin for our crise to Alaska and it was good to have because you can step out to see things or just sit out and enjoy away from the crowds. I think the above should answer most of your questions.