First night at Disney, and going straight to the Magic Kingdom. You are all ready on the right path. To get the best feel for what is in store for you, hit all the big attractions first, that's what I always do. Splash Mountain, Big Thunder, Pirates, Its a Small World (everyone has to do it), Space Mountain, Dumbo...
The natural inclination is to want to do everything at once, but its best to take it easy on the first day. Walk around, get a feel for the parks, and ride the best things that appeal most to you. There will be plenty of time to do it all.
If you want to get a good overview of the parks, take a ride on the train at mainstreet. It's really relaxing and takes you around the whole park. Also, dont overlook the People Mover in Tomorrowland.
But most important, you'll want to stick around for the fireworks at the end of the night. It's really the best way to capture the magic that first night.
Hope you have a great trip.