To tune your guitar to drop D you simply lower the 6th string (Low E) to a D.
The figure to the left is standard tuning for a guitar. The lowest string (6th) is on the far left and the highest is on the right (1st).
This tuning creates a rich D chord - play a normal D chord, but strum all six strings.
Drop D is great for power chords - barre the lowest two or three strings and you have a whole neck full of chords.