Pitch Pipes can be used. However, if you have access to a piano or keyboard you can tune to this instrument. Bearing in mind that the guitar is pitched one octave below the written note, the first string (thinnest string 'e') can be found on the 3rd note above middle C (count C as 1) and the lowest string is two octaves below middle C.

Following the diagram, the guitar strings are shown on the top stave whilst the piano double stave shows the actual pitch of each string and where to find that note on the keyboard.  Click on the number to hear the string.

Most professionals use a TUNING FORK (pitched in  'A'  44Ocps). The tuning fork has a pure sound however, it is pitched two octaves above the OPEN STRING 'A' and this can cause some difficulties for the beginner. By STOPPING the first string at the fifth fret, you can produce the same pitched 'A' as the tuning fork. i.e., 440 cycles per second.

Strike the open A string and then tap the tuning fork (on your knee) and quickly place it onto the bridge of the guitar.

Adjust the 'A' string tuning peg to coincide with the sound, not pitch, of the tuning fork. Even though there is a two-octave interval between the tuning fork and the open A string, with practice you will be able to hear the similarity. Having tuned that string, all the notes on that string are in tune.
To tune the 'D' string place a left-hand finger on the fifth fret of the A string. Strike that note with a right hand finger (using a rest stroke over the sound hole). This will produce a fuller sound than a free stroke (pick) by the bridge. Sing, hum, whistle or just remember that note. Strike the open D string and adjust that string to the same pitch as the stopped note on the A string.

Tune the remaining strings by following the fingerboard diagram below.

To tune the bass E string, you can either stop the seventh fret of the A string and then play and compare the open bass E string. There will be an octave interval between the two notes. Or you can play the open treble E and compare the sound with the bass E. (This would be a two octave interval).

1. Always tune the open string to the stopped note.
2. Always strike the open string before you adjust that string.
3. If open string will not tune exactly to the given note. Stretch the open string then re-tune.
4. Play a little tune or chord progression that you know well. If it sounds in tune then the strings will be in tune.

Having tuned the string to the correct pitch, clasp the string between your thumb and first two fingers. Pull that string (like a bowstring) so there is a distance of at least two inches from the
fingerboard and slide your hand up and down the string a few times, now re-tune that string. Repeat this action a few times before moving to the next string.

You should practice your scales and arpeggios, even though they sound grossly out of tune, you will be establishing a firmer technique that relies more on the physical knowledge of the guitar fingerboard than your aural perception of the scales and arpeggios. This form of practising also develops your power of concentration.