To tune an acoustic guitar, you will need to know the notes for each string. You should first identify the notes, starting with the lowest, thickest string. This string, when the guitar is held correctly, should be the one closest to the ceiling. From low to high, the notes are as follows: E, A, D, g, b, e
You will then need to identify the right tuning pegs for each string. You can find the tuning peg for each string by following the string up to the peg. You can pluck the string a few times and turn the peg clockwise to make it go up or counterclockwise to make it go down.
If you are using an electric tuner, such as an app on your phone, make sure you hold it close enough to the guitar that it can accurately pick up the sound. Repeatedly pluck the string and turn the tuning peg until it matches the pitch.
If you are tuning a guitar after putting on new strings, play and retune it for 15-20 minutes to accommodate any stretching that may occur.