How to find frequency of sound file ?

I am doing guitar tuner project. I want output to look like this

If i play 'B' String of guitar infront of laptop, it should detect the sound through Mike and store the sound in laptop and show the input frequency graph(Example played with frequency of 242HZ) . and it should correct with the standard frequency of the 'B' string( frequency of 246HZ).and show the output graph of standard frequency of 246HZ.

After detecting frequency it should detect Which string i have played( Example as above 'B' string) and show the difference in frequency( 246-242 = 4HZ). If the difference is less than or equal to 1HZ it should be violated.

After detecting the String , it should play the tone of that string which is having 246HZ as standard frequency for B string.

Can you suggest me how to find frequency of the guitar tone which i played and compare with standard frequency of that string ?