While it is possible to walk up to a car rental counter and rent a car, it is not recommended. Like hotel rooms, there are a finite number of cars at any rental location and making a reservation ensures that you will get a car. Without a reservation, the location you choose may be sold out of vehicles, leaving you without transportation.
All car rental companies require proof of identification before you are allowed to rent a car. Normally this proof of ID will be your driver's license because you will need a valid license to be able to rent and drive the car.
Many car companies require their renters to be over 25 years of age. Some rental companies, such as Hertz, will allow someone between the age of 21 and 25 to rent a car for an extra per-day charge. Also, there are a few (albeit these are few and far between) that will allow an 18-year-old to rent a car. Be sure to check the requirements of the car rental company you are renting from.
All rental companies will require you to have insurance to drive their vehicles. This is to make sure their vehicle is covered in the case of an accident. If you don't have your own insurance, many rental companies will sell you insurance for an extra per-day charge. Some companies will require to have your own insurance before you rent from them.
Like hotels, car rental companies like to deal with credit cards. Companies like Hertz and Avis will only accept credit cards. Your credit card must have sufficient credit on it to be able to pay for the entire car, plus extra for incidentals. (The rental company will put a hold on the funds. Should no incidentals be charged, the funds allocated for this will be released back to your card. Incidentals include road-side assistance and insurance.)
Each state has different regulations regarding rental cars, so be sure to check with your rental car provider before renting to see if you are eligible. Rental cars are not allowed to be driven into Canada at all unless you are a Canadian citizen, in which case a special traveler's vehicle permit will be required. You can drive rental cars into Mexico, but this may have distance limitations and require extra insurance, depending on the rental company.
