Plug one end of your guitar cable into the the guitar and the other end into the input of your tuner.
Turn on the power to your tuner.
Strike the first string (or low E string) of the guitar, and drop the pitch to C using the strings tuning peg on the guitar's head stock. Watch the tuner to see when you have reached C.
Tune the second string down one step from A to G.
Tune the third string down one step from D to C.
Tune the fourth string down one step from G to F.
Tune the fifth string down one step from B to A.
Tune the sixth string down one step from E to D.
Learn songs in drop C. Lots of bands use this tuning to create heavier tones. You can find guitar tabs featuring drop C tunings from some of your favorite bands. For beginners, try learning System of A Down's "Toxicity." For more advanced players, try playing "King of Nothing" by Shadows Fall.