Well, YouTube is your friend here and I agree with two of the recommendations that keep on cropping up - Enter Sandman and Killing in the Name Of. Both are awesome tracks and the latter requires a drop-D tuning which is one of the staples of metal, so well worth learning how to use it.
If you go onto one of the many guitar forums and ask about playing metal as a beginner I guarantee you will get some mouth-breathing idiot telling you that 'metal isn't for beginners' and the other phrase which crops up online often - 'learn to play'.
I say rubbish. Pick your songs, make sure your gear can get you a decent metal sound that you like and just dive in.
So, here's a couple of videos I've picked out for you which do a good job of showing you how to play some great metal riffs and properly explain it - not just someone showing off their l33t axe skillz.
I particularly like the RATM tutorial as the guy uses a semi-acoustic. But, but, but you can't play metal with a semi-acoustic, a strat, a (*insert name of guitar here that isn't one the objector owns.) Of course you can

The Metallica vid isn't the best quality, visually, but it's by far the best tutorial I found out of many on there.
(MrTwo - one of my faves too - I show it to people who say they could never learn to play anything on an instrument. If you've got a bit of rhythm in your soul and can learn four or five chords... ye can!)