Hi Cynthia,
How wonderful to be heading to Walt Disney World for both a first trip and a birthday celebration! Simply going to Walt Disney World will certainly make this a special trip for your daughter. Let me offer you some other thoughts to make it even more magical.
Be sure to pick up a complimentary "Happy Birthday" button for your daughter to wear on her special day. These are available at Guest Relations at the theme parks and at other locations throughout Walt Disney World. It is always fun to hear cast members and other guests alike wishing your little one a happy birthday! Also, although nothing extra is guaranteed, you never know when a little extra pixie dust might be sprinkled on her day. When celebrating my daughter's birthday at Walt Disney World, a cast member noticed my daughter's birthday button and brought her a huge cupcake for dessert!
Another possibility for her birthday, since you mention that your daughter loves princesses, is to book a character dining meal at which she can meet the princesses on her birthday. You can do this at either
Cinderella's Royal Table at Magic Kingdom or
Akershus Royal Banquet Hall at Epcot. Do be sure to book these as far ahead as you are able, as they are both very popular locations and may fill up quickly. There are so many more
princess experiences at Walt Disney World! Check out the link I just provided for more information. A couple of my favorite princess attractions are Enchanted Tales with Belle at Magic Kingdom and Frozen Ever After at Epcot.
I would love to help you plan more Cynthia; however, I am limited in the space that I have to answer. Please do write back if you have any further questions!