TIME the big cost for planing your own Disney vacation. Their are many great Disney authorized travel agents that can help you. Many take care of your travel arrangements, airlines, and even some will help with dining and ride reservations. I encourage people to use them for travel arrangements.
You certainly can take care of your own travel arrangements, itineraries and park touring plans by yourself. Their are some great websites and services out there with the information.
It costs you time. It will take you a great deal of time to do this, especially if your first trip or first trip in a long time. Disney is always changing. Be prepared for the changes.