Learning to tune your guitar is a skill like any other on the guitar. It takes time to perfect.
I highly recommend that beginner guitarists get a tuner - it is a lot more inspiring to play when your guitar is in tune and it actually sounds nice - well worth the few pounds one will cost...
There are several methods of tuning your guitar to itself - the 5th fret technique or harmonics - a more advanced method.
But to start with just use the notes below - note that the 1st string is the thinnest one - simply play the mp3 file below - then play the string on your guitar, then turn the tuning peg to that your string pitch goes up or down until it sounds the same as the mp3.
Might seem a little tricky at first but stick with it and soon it will be easy! You will have to listen and judge whether your string is higher or lower than the note that is played below - if you struggle with it, try singing the same note as your guitar string, and then the note that I give you below and that should help you tell which is higher and therefore which direction to tune your string!