Of the many ways to arrange the pitches of the strings of the guitar, standard tuning is the most popular. From lowest to highest the pitches are: 6E, 5A, 4D, 3G, 2B, 1E.
The fastest and easiest way to tune the six-string guitar is to use an electronic tuner. New tuners start at $25. I use a Boss TU-12H that is available for under $100.
Begin by getting the tuner to indicate the correct pitch starting on the lowest string, 6E. If the tuner indicates a pitch that is low, such as D, you must tighten the string, and conversely, loosen it if it indicates F, G, etc. Because most tuners don’t indicate octaves, you must judge by the string’s tension if you are moving it in the right direction. After the correct pitch is achieved, fine tune with the meter or LED indicator. Usually, if it points to the right it is too sharp. If it points to the left it is too flat.. If the tuner indicates flat, raise the pitch. Lower it if it indicates sharp. Most tuners indicate that the pitch is in tune when the indicator is in the middle between sharp and flat or both sharp and flat indicator light at the same time. Continue this process for the rest of the strings, first matching the note name, then fine tuning with the meter or LED indicators.
It is also valuable to be able to tune a guitar without an electric tuner using relative tuning. To tune a guitar using relative tuning, simply use the 5th fret of the string to tune the next highest one, except between G and B where you use the 4th fret of G to tune the 2nd string, B. Begin by tuning the lowest string to E with another instrument, or if none is available, get it as close as you can by ear, pitch pipe, or other reference. Then you can play an A on that string at the 5th fret and use it to tune the A string. Once the A string is in tune, repeat this process to tune the 4th string, D. Then, from the D string, tune the 3rd string, G. Play the 4th fret of the G string to tune the 2nd string, B. After the B is in tune, use the 5th fret of that string to tune the high E.
RELATIVE TUNING VARIATION
Some guitarists use a variant of this method by starting on the D string; first using the D to tune the G, G for B, and B for high E, then going back to the D string to tune the lower two strings using octaves. Sounding the D string at the 7th fret generates an A that is one octave higher than the 5th string, open A. Though these are not the same frequency, you can use the higher octave to tune the lower because you can hear them sync together when they are tuned with each other to the same note name.
After the A string is tuned, repeat this process by playing the 7th fret on the A to tune the low E string. Many guitar necks tend to bow when changing string tension, which affects the tuning of the entire instrument, so it is helpful go through the tuning process at least twice for whichever method you choose.