Hi Deborah!
Welcome to the first, of what hopefully will be many visits to Disney World! :-)
To be honest, if you have the time and money, all the parks are must see's! So, what I'll do is tell you some of my children's favorite things in each to help you decide:
Magic Kingdom
No one should miss a visit to this park, the flagship park of Disney World, ESPECIALLY on your first trip. Some of my kid's faves:
Buzz Lightyear's Space Ranger Spin-A fun ride! It is, somewhat a friendly competition, but the experience is what it's all about.
Tomorrowland Speedway-My son, at 2 1/2, was able to get on, and my daughter really enjoyed this.
Mad Tea Party and Dumbo-classic rides
And no trip is complete without a visit to Cinderella's castle and Wishes fireworks.
Epcot
Kidcot Fun Stops (World Showcase)-great craft station that your child may enjoy.
Journey into Imagination with Figment-a great attraction for kids.
Hollywood Studios
Playhouse Disney-Live on Stage!
Muppet Show in 3-D
Beauty and the Beast-Live on Stage
Fantasmic!-a terrific fireworks and light show
Animal Kingdom
Affection Section-animal petting area
Kilimanjaro safari-Best. Attraction. Ever.
The Boneyard-children loved this!
And don't forget to get a free Happy Birthday pin at any Guest Relations spot. You'll find many in the park identify right away the pin, and will shower your son with "Happy Birthday's" in the parks.
Have a wonderful, magical, memorable time!