The best time to visit Disney is between January 10-March 1 or September 5-November 1. This is their down season and there are minimal crowds, no long lines, and the weather is a bit more comfortable.
If you can, stay on the Disney property. You won't need a rental car unless you plan to leave the property because all transportation is provided for you, free of charge by Disney.
I also recommend a good, sturdy pair of walking shoes as all the Disney parks are large and there is a lot of walking involved.
My last, and probably most important, recommendation is to go to the parks early, when they first open. Then take a break in the afternoon with a nap and/or dip in the pool. Return to the parks around dinner time when the temperatures are cooling and you are rested. If not, by the evening, everyone is hot, tired, and Grumpy (sorry about the pun!).