I've never tried but it seems that most people trying to build a guitar tuner with an Arduino fail. 
As you may already know, musical instruments don't put-out a "pure" single-frequency tone. There are harmonics & overtones and that's what makes a guitar sound different from a trumpet and it's what makes two singers sound different when they are singing the same notes. That's ONE of the factors that makes it tricky to detect a note or the fundamental frequency.
From what I've read the autocorrelation method works better than FFT or FHT. And you might get better results with one of the faster versions of the Arduino.
Realistically, you are probably better-off buying a tuner or looking for a phone or computer application. Maybe you want to build your own with some "fancy features" or maybe you'll want to write your own computer application, but that's probably going to take a few weeks or longer (and it may never work perfectly) and I assume you want to start learning the guitar NOW?