You can't go wrong with a
DCL cruise if you like WDW. The cruise is better than the parks.
OK, start by choosing your dates. You can look up prices on many web sites on line. Prices vary with the date and the cabin category. Most cabins hold 4 people--with your group of 8, you'd be looking at 2 cabins. You can get them with an interior door between them or not. You might choose 3 cabins for a bit more room (you and wife in one, sis/brother in law in one, grandma and at least one kid in one, other kids wherever.) You pay adult fare for the first 2 people in a cabin regardless of age.
OK, then you decide--inside cabin, outside with porthole, or outside with balcony. In my experience, the shorter the cruise the less we use the balcony. You could even choose one or two cabins with a balcony and the third across the hall inside (which would be cheaper).
The talk of credits--some travel agents give you an onboard credit as a "thanks for booking" gift. These work just like real money and can be applied to any onboard expense, including tips. In addition, if you make ALL payments using your Disney Visa card, you would get $50 onboard credit for each cabin. You also get $25 per cabin for booking on line (your TA can book this way for you too). SO...you can get $75 per cabin from Disney + whatever more your TA may give you.
The nice thing is that the listed cost of the cruise includes everything you NEED to spend except tips (tips are $12 per person for each day of the cruise, so on a 4 night cruise you would calculate $48 for each person in your group). You CAN spend a small fortune on the ship or you can have a totally wonderful cruise and spend very little. Places that you can spend money are Shutters (photo shop) $10-20 per photo depending on size. OUCH! We use our own camera. The character attendants are happy to take family shots with your camera. Soda/milk/juice/coffee/tea are included. Alcohol, smoothies, and specialty coffees are extra...but you are allowed to bring your own alcohol if you wish. Shore excursions--prices range from a $6 tube rental on
Castaway Cay to $400 for a "Dolphin Trainer for a day" experience. You can bring your own snorkel equipment, and you can have a great time exploring islands on your own.
For a first timer, I'd suggest an expert
travel agent who specializes in Disney. I'd only deal with one who has sailed on DCL and who would speak to me on the phone before, during, and after the booking. I'd choose experience and service as more important than the amount of onboard credit or booking gift they'd give you.
That's for starters. REad here...consider the passporter, but realize that DCL is constantly changing and that any book is out of date by the time it is printed despite the best intentions of the authors. The info here is actually more current.