Get a guitar tuner. I have the SuperSnark model.
Clip the tuner on to the guitar.
Tune the thickest, low string to E.
Tune the 5th string to A.
Tune the 4th spring to D.
Tune the 3rd string to G.
Tune the 2nd string to B.
Tune the 1st string to E, one octave higher than the thickest string.