Not sure if you've had a chance to look at the online reservation system on the WDW site. Spend some time over there so you are thoroughly familiar with the resort you want to stay at and the type of room you want. You don't want to waste money and time on the phone while the CM tries to explain the different types of rooms available: standard, garden view, water view, king, preferred etc etc ad nauseum.
Know what type of ticket media you want and for how many days.
Know what type of dining plan you want (if at all)
Know if you want the Disney
travel insurance (or if you will purchase your own).
You can play around on the website to get all of this information and look at cost differences for different options.
Know if you will purchase your flights through Disney or if you will buy your flight tickets from the airline directly.
Once you have all of this thoroughly researched in advance, you'll be ready to call for your reservation.
They'll ask:
Which resort, what type of room, arrival and departure dates, how many adults and children, names of each person in the room (and ages of children). If you want a specific discount now is the time to tell them.
Then they'll ask about the type of park ticket and for how many days. Then about the dining plan. Then about insurance. Then about Magic Express. Then the big one: how will you be paying for that madame?? You'll need to give them your credit card number for the deposit. They'll tell you the date the remainder is due. They'll also want your address, phone number and email address. They should give you a confirmation number.
Make sure you ask them to email you a copy of the reservation along with the confirmation number. It is usually done automatically, but I always ask anyway.
HTH