This page details how to tune a standard six string guitar to the standard EADGBe tuning.
First, you must know which string on a guitar is tuned to which note. The low string (the biggest/thickest one) is tuned to E. The string beside that is an A. The next D. Then G, followed next is a B. Finally, the top string (the smallest/thinest one) is tuned to E as well.
Using an Electronic Tuner
For most beginners, the best way to tune a guitar is to use some sort of electronic tuner. These tuners will tell you which note is being played and indicate if it is either sharp (a little above the correct note) or flat (a little below the correct note). For electric guitars, most tuners will have a plug to be able to directly insert standard cable. For pure acoustics, tuners do have a microphone to pick up the notes you play. Be careful though, if it is noisy and you are trying to tune the guitar, the tuner might not pick up the correct notes, making it impossible to use.
Begin with the low E string. Strum that string alone and check the tuner. Use the knob on the head of the guitar to adjust that string to be “in tune” with E. From there, move on to do each string, tuning it to its correct note.
Tuning By Ear
Once you have been playing for a bit, your ear may develop enough to start being able to tune without the need for the electronic tuner all the time. I would attempt to do this soon, as it will help your ear develop enough to differentiate between “in tune” and “out of tune”.
In this, first tune the low E string some how. Use the electronic tuner for this, or use the E below middle C on a keyboard or piano. Use your ear to attempt to hear if the guitar string is above or below and adjust the knob accordingly.
Once you are satisfied that the low E string is in tune, fret the string on the fifth (5th) fret. Play that note. That is the A that the next string is when played open. Since you know that the low E string is correct, you can then listen to the differences between your 5th fretted note on the low E and the open A string. Adjust the knob until these two sound the same.
The same relationship exists for A and D, and D and G strings. G to B is a little different. The interval between those strings is not 5th. You must fret the G string on the 4th fret, not the fifth. That 4th fret is the B that the B string should be tuned to. Once that string is tuned up, you can use the 5th fret on the B string to tune up the high E string.
There are two things you can try. Instead of 5th fret of six string to open of fifth, try 12th fret of sixth to 7th fret of fifth, etc. It's the same notes, but one octave higher. This should be easier for you to distinguish.
Another thing you could try is tuning by harmonics. Sometimes people who normally consider themselves “tone deaf” are still able to hear the higher pitches of harmonics. If you do this on an electric guitar with the volume up a tad higher than normal you can actually feel it more than hear it.