Power chord basics

Root and fifth, two or three strings, usually distorted. Shape on the low E string:

E|-x-|      A|-x-|
A|-7-|  or  D|-7-|   (with the octave: add the next string at the same fret)
E|-5-|      A|-5-|

Mute the unused strings with the fretting hand. Downstrokes for punk, alternate for thrash. Add Palm muting for the chug.

A riff for practice (E, G, A):

A|-2-2-2-2-5-5-7-7-|
E|-0-0-0-0-3-3-5-5-|