One of the annoying tasks one has to go through often when playing the guitar is tuning it. You will usually have to do some tuning every time you play the guitar, especially if your strings are relatively new.
Most people tune by the ear. They will usually start from either the highest (thinnest) string or the lowest (thickest), tune it to E (or approximately so, depending on how good is their ear for that), and then one on the next strings one by one. In this method one will play the note of the higher (thinner) string on the lower one, by positioning one’s finger in the right fret on the guitar neck, and compare the sounds till both strings have the same sound in this position.
When you are not sure you can find the first E yourself, the easiest way would be to find a piano or flute, sound it, and start from it.
Some people will prefer to tune all strings by comparing to and outside source, instead of comparing them to each other as described above. They will either use a piano or flute, or some kind of simple tuner, which is actually in this case just a tone generator. They will play the tuner or flute to the right note and stretch the corresponding string till it plays the same note.
An easier method of tuning the guitar is using a digital tuner which instead of just playing the desired note, will “hear” the note you play on your guitar and tell you if it is right or not, or give you the name of the note you have just played. The screen display will show you if the note you play on a certain string is lower or higher than the desired, and by how much. When using such a tuner you do not have to rely on your own hearing at all – you act according to what the tuner shows you.
Such electric or digital tuners can be found either online – as a pc program or smartphone app, or as a small device.
The most exact digital tuners will have a pickup that can be clipped on to the guitar. The pickup lets the tuner “hear” the note played on your guitar and react to it shutting down all background noise.