I am a thirty year old metal head who has decided to pick up playing guitar. I have bought an electric guitar and amp. My question is where is the best place to start with my learning, I really don't want to start with mary had a little lamb, but i will if that's the best way to learn. I am just doing this for fun and I want to have fun learning. Any help would be greatly appreciated thanks
The absolute best place to start is with a good teacher who teaches the style you want to learn. A good teacher will provide direction and encouragement, and correct things like posture and hand position that can turn into deeply ingrained bad habits if you don't know any better... and there are a lot of famous guitarists out there that you'll be better off not emulating.
As far as what to learn for metal:
'Mary Had a Little Lamb' (and pieces like it) are the best place to start learning sight reading - they are simple, slow, and recognizable... so you catch and correct errors when you play the wrong note. Sight reading is a useful skill to have, but probably not too critical for starting to learn metal. If you might want to branch out into other styles later on, it's easier to learn sight reading from the beginning, but that doesn't mean you need to work on stuff like this exclusively.
If you want to play rhythm, start by learning the open position chords C, E, A, D, G, Am, Em, Dm, G7, E7, A7, and D7. Those 12 chords will let you play many, many songs in the keys of G, D, and A. Then add the F major and B7 chords; after that you'll want to learn barre chords... you'll also want to learn a number of different strumming patterns in 4/4 time. Although not a lot of this will directly apply to metal tunes, it will get you used to hearing chord changes, moving several fingers at the same time, accurately fretting chord forms, and playing strums with the right hand. Then learn the basic 'power chords' - together with the basic barre chords, this will let you play most metal on rhythm.
If you want to play lead guitar, start with the pentatonic scale in all positions. You'll also want to learn the common techniques for lead: bends, slides, hammer-ons, pull-offs, double stops, and maybe harmonics. You'll want to pay attention to developing accuracy with your right hand, and learn alternate picking - economy picking so you can shred will come later as you develop.
Also work on developing your ears. Listen hard to what people are doing with the guitar, and try to figure out how they're achieving it. Because the metal sound relies on distortion, this can be a challenge. On the plus side, the structure of most metal is pretty basic; you won't run into many surprises, and as your ears get better, you'll be able to hear the changes fairly easily.
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