It you were to tune a half step down, your guitar would be tuned to Eb, Ab, Db, Gb, Bb, Eb rather than standard tuning, which is E, A, D, G, B, E.
To tune my guitar down a half step, I use my electronic tuner. It has a button that will lower all of the notes a half step. If your tuner will not help you, you can hold each note at the first fret and tune the string to the correct pitch that way.
i.e. If you were to hold the E string at the first fret, tune that note to an E. In standard tuning, holding the sixth string at the first fret would give you an F, but if you are tuned down a half step, holding the sixth string at the first fret will give you an E.
I hope I didn't confuse you too badly. Let me know if you have anymore questions.