You have to buy the car first before you go to the DMV and register it and get license plates. You would go into the DMV with your new car's title. If you buy the car from a dealership, they will usually take care of all that for you, so all you have to do is go into the dealership and purchase your car. Oh, and make sure to negotiate on the price!
This answer should not be construed as creating an attorney-client relationship, and is for informational purposes only.