Need Help Programming PIC32 microcontroller

I am working on a project for college and I hit a snag in my programming. This is my first time to ever deal with programming microcontrollers and I'm not sure how to measure frequency. It has 16 analog inputs and I need to measure the frequency of 6 guitar strings. They output about 1 Volt rms and vary in frequencies from 90 to 630 hz. If anyone could help with this I would appreciate it. I am programming this in MPLAB using C.