Let me start off by saying that every guitarist should own a guitar or chromatic tuner in addition to knowing how to tune by ear.  Decent guitar tuners range from $14 up and there are lots of brands to choose from; some of the more popular entry level ones are made by Korg, Seiko, and Planet Waves.  Personally, I’d go for a chromatic tuner because they allow for more tuning variations compared to a strict guitar tuner.  However, the chromatic tuner requires more work since it’ll tune any note you play on the guitar.

Both methods explained below, assume that your 6th string is roughly in tune.  If you're 6th string is way off you'll apply too much or too little tension to the neck, which can result in neck bowing and/or broken strings.  If you think you're really out of tune, you may want to ask for help or try using the tuning link in my links section to get your guitar properly tuned.  Afterwards, you can use the following methods to quickly maintain the tuning.

Method #1 -  the most common way for a beginner to tune their guitar. 

Tuning the 5th String (A)

Hold the 6th string down at the 5th fret (A) with your index finger.  Now play the 6th and 5th strings in turn and adjust the 5th string tuner to bring the string into pitch with the fretted note.

Tuning the 4th String (D)

Hold the 5th string down at the 5th fret (D) with your index finger.  Now play the 5th and 4th strings in turn and adjust the 4th string tuner to bring the string into pitch with the fretted note.

Tuning the 3rd String (G)

Hold the 4th string down at the 5th fret (G) with your index finger.  Now play the 4th and 3rd strings in turn and adjust the 3rd string tuner to bring the string into pitch with the fretted note.

Tuning the 2nd String (B)

Hold the 3rd string down at the 4th fret (B) with your index finger.  Now play the 3rd and 2nd strings in turn and adjust the 2nd string tuner to bring the string into pitch with the fretted note.

Tuning the 1st String (E)

Hold the 2nd string down at the 5th fret (E) with your index finger.  Now play the 2nd and 1st strings in turn and adjust the 2nd string tuner to bring the string into pitch with the fretted note.

Now some guitarists move on to another tuning method involving harmonics at the 5th and 7th frets, but both this and the previous method aren’t very reliable so let’s just move to the master’s choice ;)

Method #2

There are a couple ways to perform the next method and both assume you can play harmonics.  I’ll explain how to perform the harmonic method using an E reference harmonic on the 6th string at frets 5 and 12.

With your 6th string in tune or close to it, play a harmonic at the 12 fret.  This will be your reference pitch for the 4th and 5th strings.

• Compare the harmonic you played on the 6th string to the 7th fret on the 5th string and then adjust the 5th string to match
• Compare the harmonic you played on the 6th string to the 2nd fret of the 4th string and then adjust the 4th string to match

Now play a harmonic at the 5th fret of the 6th string, this will be your reference pitch for the rest of the strings on the guitar.

• Compare the harmonic you played on the 6th string to the 9th fret on the 3rd string and then adjust the 3rd string to match
• Compare the harmonic you played on the 6th string to the 5th fret of the 2nd string and then adjust the 2nd string to match
• Compare the harmonic you played on the 6th string to the open 1st string and then adjust the 1st string to match