How to play power chords on the guitar

There are heaps of power chords, but playing power chords is just the same as playing any other chord you just strum or pick it

A power chord is a chord without a third, thus making it neither major, nor minor. If you get technical about it, power chords aren't even chords, just intervals of fifths. This allows the mimicking of regular chords on a guitar with heavy distortion in place of standard chords which don't sound so good when distorted. This should help you learn how to play them:
http://guitar.about.com/library/weekly/aa092600c.htm