Probably the easiest way is to use an electronic tuner, you can buy inexpensive clip-on tuners which are usually very good and will work in a noisy environment, or there are smart phone apps available that will also do the job. It is possible to tune the guitar without an electronic tuner this is called ‘relative tuning’ but in order to be ‘in tune’ we would need some way of making sure that at least one string is correctly tuned from which we can then tune the rest. One solution would be to use a tuning fork, it will give you a reference pitch (normally A440Hz). When you strike the tuning fork it produces the note A so you then need to figure out if your A string (5th string) is higher or lower than this note. Once your A string is in tune you can then proceed to tune the rest of the guitar by relative tuning. Play your 6th string at the 5th fret, this should produce the note A so tune this note until it sounds like the A string you already tuned. Then play the 5th string at the 5th fret (note D) now tune the 4th string to sound like this note. Next play 4th string at the 5th fret (note G) tune the 3rd string to sound like this note. Play the 3rd string at the 4th fret (note B) tune the 2nd string to this note. Finally play the 2nd string at the 5th fret (note E) and tune the first string to this note. Each string needs to be perfectly in tune before moving on to the next one as even a small discrepancy will effect every other string. Tuning this way requires practice to make sure that you can hear the differences between two pitches. As tuning is very important I would recommend beginners to use a electronic chromatic tuner and if you like double check your tuning using the relative tuning method described here, this should help you to start to recognise the difference between pitches. Have fun!