Standard tuning for six string guitar is:
  1. E
  2. B
  3. G
  4. D
  5. A
  6. E
String 1 is the high E, the small string at the top of the neck, while string 6 is the low E at the bottom. Also remember, that pitch descends by number. String 1 is a high E, string 2 is the first B below string 1, string 3 is the first G below string 2, etc. So you know which octave each note belongs to, the frequency for A on string 5 is 440 Hz (just below middle C).

Technically, you don't have to use these notes, but they are considered the normal notes so if the tablature you're using doesn't tell you how to tune your guitar, standard E tuning can be assumed. Another popular variant of standard tuning is to tune a half-step lower using D#, A#, F#, C#, G#, D# instead, although the only real rule to standard tuning is as follows:
For strings 2, 4, 5 and 6 - Each string played on the 5th fret will be the same note as the string immediately above it (1, 3, 4 and 5 respectively) played open.
For string 3 - This string played on the 4th fret will play the same note as string 2 played open.
So there you have it. Other ways of tuning your guitar include drop tuning and open tuning.
See also How to tune a guitar
A more accurate way to tune a guitar than with the frets is with natural harmonics. Frets are inherently slightly inaccurate, and can give you a slightly out of tune note. Harmonics are dependent only on string tension, not the frets at all, and so are a much more accurate way of tuning. Also, you can hit the harmonic and take your hand away, so you can tune while the note is still playing.

To tune with harmonics, for strings 3, 4, and 5, the harmonic at the 7th fret is the same note as the harmonic at the 5th fret of the string above it. So if you want to tune the 5th string, hit the 7th fret harmonic on the 5th string and the 5th fret harmonic on the 6th string. These notes should be the same. To tune the 2nd string, use the 8th fret harmonic on the 6th string. Those should be the same note, or possibly an octave apart, I can't remember. The 1st string can just be tuned to the bottom string since they are the same note 2 octaves apart.

That said, I usually tune my guitar with frets because I have a lousy ear and I find it difficult to tune higher pitches, so it's more difficult for me to tune the harmonics which are much higher pitched and I don't care that much if I'm a little out of tune when I'm just practicing. Of course, you can avoid all this by buying yourself a tuner which costs about 10 bucks and will give you accurate tuning as long as it has batteries.
