Hi Melody!
I recommend getting to the parks early! In fact, I suggest arriving at least 30 minutes before their posted opening times.
A lot of guests, including my wife if I would let her, love to sleep in. Not everybody can get to the parks when they first open. Thus crowds build during the morning hours; slowly at first but then speeding up as morning progresses. By lunch time, the parks will be busy. Sure there will be people coming and going throughout the day, but the parks really won't begin to empty out until the evening.
I have generally found this all to mean that I can experience more attractions with less stress in the morning. Four headliner attractions that I can do in the first hour might take several hours in the afternoon. I think it makes the most sense to get up early, tour a park, and then head back to the hotel around lunch time for a break, swim in the pool, nap, etc. Then you will be refreshed and ready to go back to the parks in the evening. The parks are absolutely beautiful at night.
The only exception to this "get there early" rule might be the Animal Kingdom. Please click
here to see an answer I wrote earlier about possibly touring that park in the afternoon. However, if you want to see as much of that park as possible then I suggest getting there before it opens as well!
Have a magical vacation.