DFT/FFT in C#

Hello everyone, I'm in the process of writing a console app (C#), where a
user uploads a picture, then clicks a button, and the Fourier
transform of the picture displays. I have everything set up except for
the FFT algorithm itself. My first question is what value do you get
from a pixel to plug into the algorithm? Like in the loop, when i grab
my first numerical value from the picture to plug into the algorithm,
what is it? Something involving the magnitude of the RGB values? Also,
once the FFT has been applied you end up with complex numbers. Is the
magnitude used for the new data value? Lastly, once you get your
transformed function, how do the values get translated to something
that can be redrawn(kind of relates to my first question)?

Thanks in advance for any help.