Make sure your sixth or low E string is in tune. There are a number of ways to do this if you do not have perfect pitch, including using a piano, keyboard or other instrument as a reference. Without a properly tuned low E string, you will not be able to tune the other strings on your acoustic guitar to the correct pitches.
Place your first finger on the fifth fret of the low E string. Play the low E string, followed by the open A, or fifth string. Raise or lower the pitch of the fifth string until the fretted E and open A string have the same pitch. You may have to play the notes several times before achieving the proper tuning.
Place your first finger on the fifth fret of the A string. Play the A string, followed by the open D, or fourth string. Raise or lower the pitch of the fourth string until the fretted A string and open D string have the same pitch. Again, you may have to do this multiple times to find the correct tuning.
Put your first finger on the fifth fret of the D string. Play the D string, followed by the open G, or third string. Raise or lower the pitch of the third string until the fretted D and open G string have the same pitch. Repeat the process until the notes match up as closely as possible.
Put your first finger on the fourth fret of the G string. Play the G string, followed by the open B, or second string. Raise or lower the pitch of the second string until the fretted G and open B have the same pitch. Repeat as needed until correct pitch is achieved.
Place your finger on the fifth fret of the B string. Play the B string, followed by the open high E, or first string. Raise or lower the pitch of the first string until the fretted G and open high E have the same pitch.
Repeat the tuning process, checking to make sure that no notes have slipped out of tune because of raised or reduced tension caused by the tuning of other strings. Make any necessary changes as you go.