Standard tuning is EADGBE (from low to high) , for a half step down tuning just tune each string one half step (one fret) down , so it will be D#G#C#F#A#D# (from low to high , # = one half step higher).

If you want to name them with flats "b" instead of sharps "#" , it will be EbAbDbGbBbEb ("b" means one half step lower).

Drop D tuning means to tune standard as you normaly do , then drop the Low E string one full step (2 frets) down to D.