Before you start playing the guitar, you have to tune it.

For a standard 6 string guitar, the strings should be tuned to the following musical notes, by thickest to thinnest string order: mi, la, re, sol, si, and mi or E, A, D, G, B, and E respectively. One easy way to help you remember this order is the mnemonic: Eat All Day Get Big Easy.

You may want to purchase an inexpensive electronic guitar tuner. Electronic tuners pick up the sound of the string and let you know through the display whether its too low (flat) or too high (sharp). Electronic tuners are easy to use since they show you precisely when the string is tuned without having to rely on your hearing.
In order to tune the guitar correctly by hearing, you will need at least one reference pitch to tune one of the strings (preferably the low pitch, E string). If you only have one string that is tuned correctly, you can tune the remaining ones by reference to it. Tuning forks, pitch pipes, pianos, or any other musical instrument can be used to provide the reference pitch. You can also use the free online guitar tuner on the Guitarist’s Journey website for the correct notes.

Once you have the 6th string (low-pitched E/mi) tuned, tune the following strings:

1. Tune the 5th string. It’s open sound should be A and sound like the 6th string pressed on the 5th fret.
2. Tune the 4th string. It’s open sound should be D and sound like the 5th string pressed on the 5th fret.
3. Tune the 3rd string. It’s open sound should be G and sound like the 4th string pressed on the 5th fret.
4. Tune the 2nd string. It’s open sound should be B and sound like the 3rd string pressed on the 4th fret.
5. Tune the 1st string. It’s open sound should be E and sound like the 2nd string pressed on the 5th fret.

The above methods lead to absolute tuning, where each note sounds exactly as it should on any other instrument. Without a reference pitch, you can still tune the guitar in reference to itself. This is called relative tuning. As long as you play the guitar solo you will have no problem. However, the guitar will not play in harmony with another instrument, such as a piano, since it is tuned differently.