On arrival you pick up your baggage and continue following the "exit" signs. This automatically takes you past the car rental companies. They are all located cheek by jowel , and each, no doubt has its own adherants. If you have already a reservation, then just pick your suppliers desk and join the inevitable queue.
If you want a car and have not reserved then these are the companies operating there. As with most things in lifeyou get what you pay for. An older cr will invariably be cheaper to rent, and you need to watch for hidden extras or other things like prohibition of driving outside the Provence of Valencia
Or try the price comparison sites like