Metallica's guitar sound, and heavy-metal guitar sounds in general, depend upon lots of distortion, as well as lowered guitar pitches. This is a direct result of the influence of Black Sabbath. Sabbath's guitarist, Tony Iommi, lowered the pitch of his guitar because of a hand injury that damaged his fingertips. The lower pitch allowed him to play the guitar more easily. In other words, he created the sound that defines heavy metal guitar by accident.
Put together the right equipment. Marshall and Mesa-Boogie amps are ideal for producing the tone and sound necessary for playing Metallica songs. You should also purchase a distortion pedal. There are a variety of models available, which range from mild to extreme levels of distortion. The Ibanez Tube Screamer is a very respected and relied upon distortion pedal. Heavy gauge strings (.013 to .053) add the finishing touch.
Purchase a chromatic guitar tuner. Metallica uses various lowered pitches, ranging from a half step to two full steps, in their song repertoire. The most accurate and quickest way to match their tunings is with a chromatic tuner.
Lower your guitar tuning a half step to E-flat standard (the standard tuning for a guitar is E standard, or, going from the 6th to the 1st string, E-A-D-G-B-E). Plug your guitar into the tuner and lower each string half a step. Some chromatic tuners indicate the correct pitch with a green light and use red lights to indicate when the string is sharp or flat. Other tuners indicate the proper pitch with a needle that aligns with a 440 mark in the center. Start with the 6th string. Tune the 6th string to E-flat, the 5th string to A-flat, the 4th string to D-flat, the 3rd string to G-flat, the 2nd string to B-flat, and the 1st string to E-flat. Play an E chord (which is now an E-flat chord) and check the tuning.
Tune to C standard tuning. C standard tuning is two whole steps below E standard. This produces an extremely heavy guitar sound. Tune the 6th string to C, the 5th string to F, the 4th string to B-flat, the 3rd string to E-flat, the 2nd string to G, and the 1st string to C. Play an E chord ( which is now a C chord) and check the tuning.
Practice playing in the different tunings and diagram the chords. The chord shapes and fingerings stay the same but the chords obviously are different. Write down the names of the chords on a piece of paper. Start by learning all the chords in the first position. The E-flat tuning is easy to remember. All the chords are flatted: E is E-flat, A is A-flat, and so on. The C tuning is a little more difficult. The E shape is now C and the G shape is now B-flat. Take the time to learn all the chords.