So I want to setup a light system in my room to turn on when it is getting light outside, and turn off when it is dark outside. I know it is going to involve a light diode/transistor thing - but other than that I am not sure how I should go about it or how to turn on say... 120 watts at 120V, etc - with an obviously millivolt response from the light? Some sort of electronic switch?

I assume I'll need this: http://en.wikipedia.org/wiki/Switch#Electronic_switches
and this: "Vishay Semiconductors Photodiode" -- or what would it be called? I can't find the right thing I don't think? seen em' used in coil guns before....

Anyways - I know very little (essentially nothing) about electronics but would like to make this - anyone want to chime in and spoon feed or tell me any troubles I am going to be running into?  

I want to do this for a few reasons:
1. helps me stay normal (instead of going to bed at 6 AM, waking up at 2 PM)
2. I think it'll help my salvia plant flower at some point (be it fall this year, or spring next)
3. I am starting a new project which is attempting to breed northern slimy salamanders and I assume daylight/ratio hours are important for breeding or something... (probably won't happen due to the fact that there is very little recorded history of these breeding in captivity but whatever)

And opening a window is not an option

The first circuit diagram "Using a transistor switch with sensors" on the page Wizard X posted should work fine. You will Just need to connect an AC switching relay as the load instead of the LED and 470ohm resistor. If you have/can get a breadboard it would be easier to test the circuit out on that first before soldering it up

The easy way would be buying two kits online. One Light sensor and one relay kit.
Relay kits have the needed transistors for drive by the light sensor board.
If needed I can consult an associate for a guide/schematics on a diy version.