Full Step Down: D G C F A D
There are many methods to get into a guitar tuning that is a full step down, so I encourage you after learning this simple 3 Step Method to experiment with your own.
Step 1: Tune your Low E to the D (low E is the thickest string)
Tune the low E to a D. Exactly like Drop D guitar tuning. Pluck the 2 strings together or alternate between them until they sound alike. The low D will be an octave lower then the D you are tuning from, but they are the exact same note.
Step 2: Tune the rest of your guitar as you would regularly tune it
Tune the rest of the strings in relation to the D you just dropped down. Just like you would with Standard Guitar Tuning. The only thing that changes are the note names. So the tuning we are looking for is D G C F A D. If you tune the 2nd thickest string (which is normally A) to the 5th fret of the D string you just tuned, that would be a G.
The Low D String:
Then once you have the G, the 5th fret of that string becomes a C, which is the next string you need.
The tuning pattern from the top down for Standard Guitar Tuning is always 5th fret, 5th fret, 5th fret, 4th fret, 5th fret. If you wanted to tune down half a step you would tune your thickest string which is usually a Low E to the 1st fret of the D string. Or otherwise known as D#. A half step is one note below the note you start with, it is also classically referred to as a semi-tone. 2 notes are equal to a full tone or full step.
Just a word of advice to any beginners, if you wanted to tune you guitar higher say up a full step or half-step, you would be wise to use a capo, as your strings may break otherwise.