Ok it´s nice to see you playing such a legendary riff as Iron Man, made possible by the great Tony Iommi! Let me give u a piece of advice though on how u could play just like Mr. Iommi does. First of all u should play i in power chords and start of at the 7th fret on the lower E string. If u wanna make it even more monstourus after that, detune your guitar to C# and ure ready to launch one of the greatest heavy metal riffs ever made! Best of luck and rock on!
