how to get the main frequency from digital data

I want to build a machine that I put into her audio (frequency of numbers) by a microphone and converts it into digital information and then converts it to the frequency domain and then I'll filter it and recognize the two main frequencies that make up the signal and accordingly I will write down the number that two of these frequencies represent.

I was able to build a machine that converts the audio signal to digital but I do not know how to convert it in the frequency domain.

Can anyone help me or direct me to a source of information?

I work in quartus with the device of ALTERA DE2.