Most probably you already have selected a rental car and made a reservation. If you haven't, this article might be of help to you: Some tips when making your rental car reservation.
So you have waited patiently for the first day of your rental car reservation to come and it's finally here. Now you have to go to the chosen car rental company to sign a contract and finally pick up your rental car. Prepare your personal documents (id and driver's license) as well as the money amount for the entire rental period plus deposit. Most car rental companies accept electronic payments like bank transfers and card payments. If you have made a bank transfer you have to take the receipt with you. If you have prepaid part of the money for the rental car reservation, you have to subtract it from the total amount.
In the car rental company's office you have to sign the aforementioned contract for the rental car. He looks much like the rental car reservation form you have received, but this one has all your personal data, the car rental company data as well as the full rental car details. At this point you can ask the car rental representative about any active promotions and discounts you might take advantage of. It has been several weeks since you made your rental car reservation (supposedly it was not made last minute). Another document you have to sign is a protocol of acceptance. An advice from me: do make a thorough examination of the rental car using this protocol as a check list, to verify that the actual and the described protocol condition of the car match. Do this together with the car rental guy. This protocol usually states any existing damage on the paint, tire types, fuel quantity, etc. The purpose is to write down the actual condition of the car on pick up in order to be able to compare it with its condition later on return. You have to pay for any additional damage and missing stuff from the rental car. This is usually covered by the deposit you leave when you pick up your rental car. That's why you have to be extra careful examining the car so that you don't have to pay later for existing damages you have overseen and left out of the protocol.
After you've examined your rental car, signed all documents, made the payment and received a receipt you are ready to take your rental car and hit the road.