Oh, you both are going to have so much fun. Disney is my favorite place to go, it is so wonderful and magical. It is going to be great, especially since you both will be experiencing it for the first time.
My number one recommendation for you would probably be to take her to Cinderella's Royal Table or Cinderella's Gala Feast in Grand Floridian, and then also Princess Storybook in EPCOT. They are kind of costly, but she will never for get it. Especially if she loves princess', this will be great. Cinderella's Royal Table is actually in the castle, but probably my least favorite as far as food. But the others are very nice. There are lots of character meals through out Disney that you can find, another of my favorites is Chef Mickey's at the Contemporary, it is really cute and has the fab 5. If you plan to book a lot of dining, you should look into a travel agent that can take care of these for you. My travel agent just has me write down what I want and then she books it. Some of the more popular ones sell out really quickly and she has always been able to get them if I get the dates to her. It's really nice not to have to worry about them.
Other than that you have lots of options for her. Magic Kingdom and Animal Kingdom will probably be her favorites as they are geared to a slightly younger crowd. But all the parks have a little something for everyone. Magic Kingdom is probably the best for younger kids. Cinderellabration would probably be something she would really love, as are all the parades and fireworks, they are great to see.
I think this pretty much covers everything. The main things are going to be different for each person, but there is a lot to do. This is a great site to plan your trip: http://mousetrips.com/WaltDisneyWorld.html It's also the travel agency I use. They have lots of information on the parks, as well as rides, height requirements, kids programs, etc. You can find a lot of really useful information there.
I hope this helps. If there is anything I didn't answer you can e-mail me, I'm happy to help.