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