In order to begin tuning the guitar, you'll need a "reference pitch" from another source. Once you've found a source for this initial pitch (it could be a piano, a tuning fork, another guitar, or any number of other options), you'll be able to tune the rest of your instrument by using that one note.
Without a reference pitch, you can tune your guitar, and it will sound fine on it's own. When you try and play with another instrument, however, you will probably sound out-of-tune. In order to interact with other instruments, being in tune with yourself isn't enough. You'll need to make sure that your E note sounds the same as theirs. Thus the need for a standard reference pitch.
Now that we've got our sixth string in tune, let's move on to learning how to tune the rest of the strings.
- Make sure your sixth string (E) is in tune.
- Play the sixth string, fifth fret (A), then tune your open fifth string (A) until it they sound the same.
- Play the fifth string, fifth fret (D), then tune your open fourth string (D) until they sound the same.
- Play the fourth string, fifth fret (G), then tune your open third string (G) until they sound the same.
- Play the third string, fourth fret (B), then tune your open second string (B) until they sound the same.
- Play the second string, fifth fret (E), then tune your open first string (E) until they sound the same.
- Verify the tune repeating all the passages and playing some simple chords like C Major, A Minor, etc., listening to the results.