Before we get started make sure your tuned up. Next grab your distortion pedal, doesn't matter what kind just a long as it growls back at you. This is what gives you that cool heavy sound. Ok lets roll on. First place your first finger (Pointer Finger) on the 3rd fret E string/big string. If you don't know what fret this is just count from the guitars nut until you reach the third fret or just place your finger on the 3rd box on the guitar neck.
This is usually the first dot or square on the guitars fretboard. Next place your 3rd finger on the 5th fret second string which is the A string. Now strum/pick just those 2 strings You have just played a G Power Chord, also called a G5 chord. These chords are used alot in rock/metal styles. A power chord uses the 1 and 5th intervals of a scale that's why they call it a G5,A5,F5 and so on.
Tip - Keep the same pattern just move it around up or down the guitars fretboard.