There are many ways to tune a guitar, including electronic tuners, pitch pipes and tuning using a piano. So many options can be daunting in the beginning so I have laid out one of the standard methods, relative tuning. This means to be able to tune the guitar in relation to itself. At the bottom of the article you will see a diagram illustrating the method, but I will explain it verbally first.
First of all this method has limitations because if the first string of the guitar is not up to concert pitch (the standard pitch that every instrument in an orchestra would tune to) then the guitar will sound in tune to itself but not in comparison to other instruments.
Here is how we relatively tune the guitar:
1) We start on the thickest string (the one closest to us, the 6th string), press down on the 5th fret and compare the pitch of that note to the OPEN string above (5th string). It should be the same pitch. If not, then tune the open string up or down according to the pitch of the fretted note. Once the open 5th string is in tune to the fretted 6th string you can proceed to the next string.
2) Now we repeat the exact same process on the next two strings. Press on the 5th fret 5th string and tune the open 4th string to that note. Once the open 4th string is in tune to the fretted 5th string you can proceed to the next string.
3) Now we repeat the exact same process on the next two strings. Press on the 5th fret 4th string and tune the open 3rd string to that note. Once the open 3rd string is in tune to the fretted 4th string you can proceed to the next string.
4) These next two strings are where we see a slight change in the fret we tune from. Press on the 4th fret 3rd string and tune the open 2nd string to that note. Once the open 2nd string is in tune to the fretted 3rd string you can proceed to the final part of the tuning process.
5) Press on the 5th fret 2nd string and tune the open 1st string to that note. Once the open 1st string is in tune to the fretted 2nd string you are done!
You may need to check the tuning 2 or 3 times before the tuning is stable. Learning to tune this way will not only give you an easy way to tune, but it can help improve your ability to hear and identify tones little by little. Enjoy!