Photoresistor and another switch I am using the circuit diagram below to turn ON/OFF the lights using Photo-resistor. 

How can I use the Arduino to bypass the Photoresistor effect?
Let say I want to turn ON the lights even though Photoresistor doesn't want to.

Take note the 12V source is required by the LED. Knowing that Arduino must work with max 5V.
Be careful with the 12V, the Arduino can not handle 12V at its pins.

The input can be the photoresistor with a resistor to 5V (not to 12V). Use a analog input to measure the amount of light.

The output can be with a resistor to the base of the transistor.
This is an example: PighiXXX transistor
Any unused digital output can be used.
Why do you need 12V for the led ? Is it a special led ?