Assuming Orlando.
If you are staying on Disney property, they have a bus from the airport to the properties. They also have a big bus system so you won't NEED to rent a car. There is a car rental place on Disney property called the Disney CarCenter. If I remember right, this is Alamo and Budget (one location). You will need a credit card in the name of the driver in order to rent a car. My inlaws tried to pay for the car I was driving and we were told NOPE!
http://www.mousesavers.com/onedayrentals.html
The airport has a ton of rental places right at the airport, with return at the airport. You can get a list off of the airport website.
http://mcocarrentals.com/?gclid=CNCFxMTb-acCFYLc4Aod_1y8qQ
The Orlando airport is on a toll road. 2 tolls for a total of around $3.00 (i think). You are responsible for paying any /all tolls if you are driving. The cars do have the toll pass on them, but you will get 'fines' in the mail when you get home if you use these. A cop had told us that we could since it was a rental. Then a week after we got home, we got two 'fines' with $5-6 processing fees. I called them up. The cop said ... they said he was wrong. So $15 in fees and fines for a $3 total toll...
M.