Set your guitar to release D. On the low E-string, start loosening the string until it has dropped a full step. Press down on the 7th row of the low E to match it with the A string.
When you have a guitar in drop D, you can play power talks very easily. By holding down the three lower strings on the same fret, you can make simple chords. Hold down all three low strings on the fifth fret for four strums, then just let go and play four strums on the open.
Songs that use the open, fifth, and 7th frets are probably the easiest to learn. Practice playing a song that uses each of these positions for a while before you start adding more chords.
Drop D tuning is a very flexible and capable tuning for a guitar, as well as being very light for power cords if you are in a rock band. This guide shows you how to play a guitar correctly in drop D.