1. Tune one string to a note from another source. You could use another guitar (one that has already been tuned), a piano, or somebody that knows how to tune could guide you along. Another source is like the one on the right--an online guitar tuner.
2. You now adjust the strings to match the pitch of the source, which in this case, is the online guitar tuner. Click on a key on the online guitar tuner then find the correct tuning gear (see guitar headstock image on right) for the string and then turn the gear slowly in one direction or the other. After about half a turn you should hear the string change pitch either up or down. This will tell you which way you have to turn the gear to tighten the string (to raise the pitch) and which way to loosen the string (to lower the pitch).
Now compare the sound of the string with the sound of the online guitar tuner. If the string is lower than the online guitar tuner, tighten the string to raise the pitch. If the string is higher than the online guitar tuner , loosen the string to lower the pitch.
Go slowly. Do not turn the gear rapidly. Turn about a quarter of a turn and then compare the string to the online guitar tuner again. You'll probably have to repeat this process several times. When the string sounds close to the online guitar tuner make smaller turns.
Learning to tune your guitar takes time. Some things on guitar can be learned in minutes, some in days, and others in weeks, but tuning will sometimes take even longer, because you have to train your ears. If results come slowly or don't seem to be making any progress, don't be discouraged, just keep working at it.
Learning to tune your guitar is learning a new way of using your hearing.
Thanks to http://www.howtotuneaguitar.org for the Online Guitar widget