One of the best and most accurate ways to tune your guitar is to use a digital tuner. You can get one for a pretty low price these days and I guarantee you its going to be one of the best investments you make for your guitar playing.
Another method is the Relative Tuning Method. If one of your strings is already in tune, its possible to use this method. Lets assume that the 5th string (A) is in tune. (you can use a pitch fork to get this A string tuned) Use a left hand finger to press down on the 5th string at the 5th fret. This note gives you a D note, which is the same note as the open 4th string. Play the open 4th string and adjust the pitch until you get the same pitch as the fretted 5th string.
Once the D string is in tune, fret it at the 5th fret and this will give you a G note. This should sound the same as the open 3rd string.
When your G string is in tune, fret it at the 4th fret to give you a B note. This gives you the same sound as the open 2nd string.
Next, fret your 2nd string at the 5th fret and this will give you an E note, which is the same note as the 1st open string.
The diagram shows you the relative tuning method.
Also check out http://www.howtotuneaguitar.org/ for more info.
Cheers and stay in tune !!