I think it would be difficult to get a PLL to lock on to the fundamental of analog signals such as an electric guitar generates, especially considering the varying frequencies generated by the strings. I am not a guitar player, but I am guessing that, because of the nature of the pickups, an electric guitar would have much higher harmonic content than an acoustic one. Perhaps if your PLL had a VCO with a narrow deviation range, and you pretuned it to the expected fundamental frequency for each string, you might get it to work. An acoustic guitar might be much easier, but I don't really know.
I thought you were trying to extract a fundamental sine wave from a square wave of varying frequency, which is a different problem. You should always give as much information as possible in your original question.
Your best bet for a general solution would probably be a digital signal processor (DSP) running a fast fourier transform (FFT) algorithm, but I can't see anyone doing this in a week, even if he were experienced in digital signal processing.
Have you already tried to make a tuner but failed? If so, what did you try? Why did it not work? This will give us an idea of what doesn't work, so we don't duplicate you mistakes.