Very easy. Ring any car rental outfit. Or your hotel reception will arrange it. You need a credit card and a driving licence. If it's the old non-photo-type they will probably want to see your passport as well.
However it will be cheaper to book it here before you go. I can't see you deciding you don't need one.
Although the basic rental costs in Florida are very cheap, add ons in the form of insurances, etc, will bump up the cost enormously. Booking here gets all that included at reasonable cost.