How do I connect an Arduino board to an RC Car?

This is really a robotics question and you have provided too little information. If it is an RC car then it has to have wiring for connecting the receiver to the power controls or for adding an ESC (electronic speed control) The Arduino almost certainly does not have the drive capability for the electronics on the car, so you will have to provide buffer chips and exactly which are needed will depend on the car choice - which is a lot more complicated than we can answer here - find a local RC club that does cars or robotics forum or MakerSpace to see if you can get direct help.