All guitars including electric, acoustic and classical use the same standard tuning also know as Concert tuning or Concert pitch.
There are several methods available to tune a guitar such as with a tuning fork, guitar tuner or to a keyboard instrument such as a piano. If you play in a band with a keyboard player you will likely tune to the keyboards.
Regardless of the method you use to tune the end result will be that the bottom 5 open strings will sound the exact same as other notes played on specific frets on the top 5 strings as shown in the picture below.
In other words when you play the; 5th fret on the 6th string it will sound the same as the 5th string open. 5th fret on the 5th string it will sound the same as the 4th string open. 5th fret on the 4th string it will sound the same as the 3rd string open. 4th fret on the 3rd string it will sound the same as the 2nd string open. 5th fret on the 2nd string it will sound the same as the 1st string open.
This is very important to know since you can tune all of your strings once you have at least 1 string in tune. You will also need to know this if you choose to use a tuning fork as mentioned below.
Guitar tuners are easy to use and work on both Electric and Acoustic guitars. For electric guitars you just connect your guitar cord directly into the guitar tuner input jack. For Acoustic guitars the tuner has a microphone so either lay your guitar flat and place the tuner on top OR position your guitar close enough so the tuner can pick up the sound.
You simply turn on the guitar tuner and strike a string. The tuner will know which string has been played and will tell you on the display. Once the needle on the display is straight up you string is in tune. The needle will also show you if a string is flat (tune the string higher in pitch) or sharp (tune the string lower in pitch). Continue until you have tuned all 6 strings.
Tuning To A Piano/Keyboard
In order to tune to a keyboard or piano simply find Middle "C", strike the key that matches an open string and hold the piano/keyboard sustain pedal down while tuning your guitar string until they sound the exact same. Do the same for each of your strings as shown below.
The Piano Sustain Pedal is located on the Far Right of the 3 pedals.
An electric Piano or Keyboard with 1 Pedal is usually the Sustain Pedal.
Tuning With A Tuning Fork
You will need to obtain a tuning fork pitched at A "440" which is the same frequency as the 5th string played open. To get the tuning fork to sound you need to strike it on a soft surface such as your knee. NEVER strike a tuning fork on a hard surface or you will damage it such that it won't vibrate at the correct pitch.
When the tuning fork sounds, tune your 5th string open to the exact same pitch as the tuning fork. Once you have the 5th string in tune you can put away the tuning fork and you must now tune the other strings as shown in the first diagram at the top of this page. Start with the 6th string by tuning the 5th string open to the 5th fret on the 6th string. This means that you will be playing 2 strings at once while tuning the 6th string.
With the top 2 strings in tune proceed to tune the 4th string open to the 5th fret on the 5th string and so on until you have tuned all of the strings.
Another method involves the use of harmonics once you have the 5th string tuned but this is more advanced and a little tricky to do properly.
There are other types of tunings besides Concert tuning known as "Open" tunings where you tune the strings to an open chord such as a G major. To see the various other tunings refer to the GUITAR CODEX.