Hello,
Before you start FM, you should have "heavy" studies on AM, transistors, diodes, amplifiers, rf...
Every radio has a detector.
What is a detector?
Detector is what "extracts" the audio frequency from the modulated radio-frequency waves.
To detect AM is quite simple, because it's amplitude modulated and the frequency is always the same.
FM is more complicated, the amplitude is aways the same, but the frequency always changes according to the AF modulation.