The top five things I might recommend to someone going to Disney for the first time are different than my own top 5, so
Top 5 recommended:
-At MK, you must enjoy Main Street (that means not running through it to get to the E-rides without even stopping to look). There's fastpass, so you don't need to rush so much. If you have a couple days to open MK, choose one to hit the rides, and choose one to enjoy the ambiance.
-At DAK, you must do the Safari (preferably in the morning, first thing)
-At EPCOT, you must visit World Showcase and stop in each country and take some time to enjoy the architecture, food, shops, and drink
-You must understand that you will not do and see everything ever no matter how many times you go. Live in the moment.
-You must see either Wishes or Illuminations (my vote is STRONGLY for Illuminations but for a first timer, Wishes is the quintessential fireworks over the castle that I know you want to see.)
Top 5 things I must do:
-Eat a chocolate covered frozen banana while waving to the Tree of Life.
-Ride the safari first thing in the morning.
-Enjoy Soarin' several times.
-Enjoy Spaceship Earth as many times as possible (this is the ride that screams "Disney" to me. My first time there, I rode it. When I worked there, if I got downhearted, I went on that ride and it reminded me why I was there and why I love the place).
-Enjoy Illuminations as many times as possible.