A power chord is played with a limited number of shapes, and what chord you play is determined where on the neck you play the chord. Below are two examples.

An important thing is to mute the notes not in the chord, this can be done in various ways. Below is an example muting the strings with your index finger by just holding the finger over all strings, so the only notes you hear are hold by your middle and pinky.

Power chords are often played with the root on the low E or A string, below are a few examples of power chords in different places on the fretboard. Check this to learn find the notes over the entire fretboard.

Here is another chart on how to play common open chords, also see the preset-menu for more guitar charts and tutorials.