I want to make a program that can take sound input from a microphone, and determine if the sound is one of three pitches (or maybe decibels).

Can someone please help me to write a C++ program which could identify and process various characteristics(frequency, amplitude) of the sound from the mic input..

You're going to need a library, there are no standard C++ methods for this. See http://www.mega-nerd.com/libsndfile/ I have never used it, but I'm willing to bet that a few people around here have.