Metal is a very broad genre of music and is often misunderstood, so instead of teaching death metal or thrash, I teach the basics of most metal sub-genres.
Keeping your guitar tuned is a must if you want to sound good, but changing your tuning can really change the sound of your guitar! Metal is not defined by a specific tuning. Some people say that you need to play ion certain tunings to play metal, well I call B.S. Use what ever one you prefer and you don't have to limit yourself to one tuning.
Some of the tunings are:
(Tunings are in order from Highest to lowest, and strings are in order from thickest (low E or 6th string) the highest (high E or first string))
Standard (E, A, D, G, B, E) (Alice In Chains*) *maybe
Drop D (D, A, D, G, B, E)
Half-Step Down (D#, G#, C#, F#, A#, D#) (Metallica)
Drop C# (C#, G#, C#, F#, A#, D#)
D Tuning (D, G, C, F, A, D) (Children Of Bodom)
Drop C (C, G, C, F, A, D)
C# Tuning (C#,F#, B, E, G#, C#) This is my favourite
Drop B (B, F# , B, E , G#, C#)
C Tuning (C, F, A#, D#, G#, C) (In Flames)
Drop A# (A#, F, A#, D#, G#, C)
There are more tunings that go lower, but I'll leave those for you to discover yourself.
Palm muting a real simple technique that gives your playing a bit more of a percussive sound. You do this by lightly muting the strings with your strumming hand. Here is an example I used from when I first learned how to palm mute. (It also works with drop tunings) (P.M.) means palm mute).
This is a very popular technique across many genres.