Tuning down your guitar means to tune each string down to a different pitch. When you tune down a half step, you tune each sting one half step lower than the standard tuning (A half step is the same as 1 fret). This means going from
E A D G B e to Eb Ab Db Gb Bb eb ('b' means 'flat', which means a half step lower than usual).

This sounds easy but without a chromatic tuner it can be difficult. Here is a technique I use to tune down a half step...

· Fret the 1st fret of your low E String. Tune this note to an E.
· Fret the 1st fret of your A String. Tune this note to an A.
· Fret the 1st fret of your D String. Tune this note to a D.
· Fret the 1st fret of your G String. Tune this note to a G.
· Fret the 1st fret of your B String. Tune this note to a B.
· Fret the 1st fret of your high E String. Tune this note to an E.

Once you have done this, your open strings will sound the notes Eb, Ab, Db, Gb, Bb, and eb and you are now tuned down a half step! This is particularly useful if as a vocalist, you struggle to sing in the key of E. If it is easier for you to sing in the key of Eb, I would definitely reccommend this.