I am wanting to use the standard 12v that comes from the car cigarette lighter port and then use the arduino pins to act as a switch turning that 12v on and off at certain points when i take it to a light at the rear of the vehicle, please help with the circuitry.
at certain points
At certain points/locations around the vehicle? At certain points in time? Under certain conditions?
it to a light at the rear of the vehicle
What’s the wattage or current rating of the light? The simplest solution is a solid state relay. Solid state relays that can be driven directly from the Arduino’s 5V output are common. If you get a solid state relay, make sure it’s designed to switch DC. (Unlike regular relays, an AC solid state relay won’t work with 12VDC.)
A regular electro-mechanical relay would work, but most relays can’t be directly driven by the Arduino, and you’d need a [u]driver circuit[/u].
A transistor or MOSFET is another possibility. (A MOSFET is probably the “best” solution, but a relay is generally easier for a beginner to wire-up.)
Note that the Arduino’s ATmega chip operates at 5V. You can power the Arduino board with 12V through the barrel jack (using it’s on-board voltage regulator) but any inputs control signals/voltages from the car’s electronics or switches must be knocked-down to 5V or you can damage your Arduino.