Read a lot here on the Disboards, maybe think about using a Disney Specialist
travel agent (TA), and get the book "The Unofficial Guide to Walt Disney World" (the 2010 version should be out, but the 2009 will still work as well).
If you book a package (meaning room, tics & possibly dining) then you have a $200 deposit, with final payment due 45 days prior to travel. You can make payments along the way if you choose.
If you book a room only reservation, you need to make a 1 night's payment for deposit, with the remainder due at checkin. You can buy tickets from a third party dealer, but you won't be able to do the dining plans (these are only able to be added to a reservation if you purchase tickets & make your ressie a package).
Look for good deals. Right now the buy 4 get 3 free deal is out there (it is for packages... and if you stay at a value resort it is buy 5 get 2 free). Room only discounts should be coming out before too much longer. A Disney TA will keep an eye out for any great deals to come along to help keep your costs down.