Hi Kim!
This is a great question. You can rent a car at Orlando International Airport if you are flying in, but you can also rent a car at several locations within Walt Disney World.
The Walt Disney World Car Care Center houses both Alamo and National offices onsite. It is located near the Magic Kingdom and provides shuttles that will pick you up from any Walt Disney World Resort hotel and take you to the Car Center to pick up your car.
The Walt Disney World Dolphin Hotel, located on Crescent Lake between Epcot and Disney's Hollywood Studios, is another location on property for both Alamo and National. If you are staying in a Epcot area Deluxe Resort, you can just walk to the Walt Disney World Dolphin.
There are also rental car offices in the various independent hotels in Downtown Disney. You can easily get to one of those hotels by taking a bus from your Disney hotel to Downtown Disney and then walking across the street.
Enjoy planning your magical Disney vacation, and please write back with more questions!