UInderstanding FFT or DSP

[Sismetic]

Hi, Im trying to do a screen-flashing application, that flashes the screen according to the music(which will be frequencies, such as healing frequencies, etc...). I already made the player and know how will I make the screen flash, but I need to make the screen flash super fast according to the music, for example if the music speeds up, the screen-flash will flash faster. I understand that I would achieve this by FFT or DSP(as I only need to know when the frequency raises from some Hz, lets say 20 to change the color, making the screen-flash).

But I've found that I understand NOTHING, even less try to implement it to my application.

Can somebody help me out to learn whichever both of them? My email is [e-mail]. I really need help, I've been stucked for like 3 days not coding or doing anything at all, trying to understand, but I dont.

PS:My application is written in C++ and Qt.

PS:Thanks for taking the time to read this and the willingness to help.