Go to a couple of the "big" travel sites, such as travelocity.com, expedia.com or orbitz.com. Simply select "car rental" and enter your criteria. Each site will bring back results, and this will give you an idea of the rates that are being charged.
Once you've reviewed the rates, decide which rental car company you wish to use. Price may not always be the best factor. Evaluate any specials, upgrade offers and mileage charges. Some people are more comfortable sticking to a national brand, such as Enterprise, Hertz or Avis. You may save more money, however, going with a local or smaller company. Try carrentalexpress.com, a site that compares prices of independently owned car-rental agencies.
After deciding which rental car company you would like to use, go directly to that company's website and perform the same search. Often, the rates will be less expensive if you book directly on their site.
Contact your travel agent. Travel agents receive special deals and pricing that may not be found on the Internet. You also have the added benefit that if you need assistance or if something goes wrong while renting the car, the travel agent can be your advocate.