Before you buy tickets, you need to do is figure out how many days you will need. Then, check
HERE to see what the tickets will cost. For each day of ticket you purchase, you can visit one theme park. If you want to visit more than one theme park per day, you will need to add on the Park Hopper option. You will notice that the more days you purchase, the "per day" price drops.
Once you decide on your tickets, you can order them online on the disney website, you can purchase them at your resort or at the gates of any theme park. Tickets can also be purchased at Disney Stores or at a AAA office.
You may find it easier to book your entire package at once.
You can book this on disneyworld.com or by calling (407)W-DISNEY. You can include park tickets, hotel, even dining!
Disney World is approximately 43 square miles with 4 theme parks, resort hotels, water parks, golf courses and more. While there are some locations you can walk to, most require your driving to your destination or using the convenient Disney transportation system.
Have fun planning!