I have a 9v battery that is only needed to output power during night time/when the sun is down, how can i do this?
This is a simple PNP circuit with a light dependent resistor. When the sun goes down the resistance of the LDR increases and the voltage drop across the LDR  forward biases the PNP transistor and illuminating the LED or whatever you like within the operating range of the transistor. 

This should be a good start    



Here is another circuit that I found at
http://www.talkingelectronics.com/projects/SolarLight/SolarLight.html