To get that "rocky feel" the "power chords" are actually called "5ths" - e.g. A5, D5, etc.
A normal chord is made up of a triad of notes (i.e. 3 notes)
To keep things simple we will stick to chord of C
A "C Major" chord would have the following notes: Root = C, Third = E, Fifth = G - this creates a "happy" sounding chord
A "C Minor" chord would have the following notes: Root = C, Flattened Third = Eb, Fifth = G - this creates a "sad" sounding chord
The rocky "5th chord" (often referred to as a "power chord") is made up of only 2 notes - the root note of the chord and the 5th note
As this chord excludes the 3rd note it can work well over a major or minor chord so you can move it around the fret board quite easily - if in doubt play a 5th chord is a good get out of jail card when depping in a band!
To achieve what you want you need to learn different positions on the fretboard to play these types of chords
Sticking with the C example you can play a C5 chord in the following positions:
Root = 5th string 3rd fret
5th = 4th string 5th fret
Root = 6th string 8th fret
5th = 5th string 10th fret
Root = 4th string 10th fret
5th = 3rd string 12th fret
Root = 3rd string 5th fret
5th = 2nd string 8th fret
Root = 2nd string 1st fret
5th = 1st string 3rd fret
As long as you keep the shapes the same you can move them around the fretboard to get the desired chord and “sound” you want
Rock songs are often made up of layers of these types of chords to give that “big sound” with each different layer being EQ’d slightly differently and different effects, levels of overdrive etc.
I find that having the root note on the 5th string gives a good “tone”
The other thing to bear in mind is that some bands tune the guitars down a bit, which can give a characteristic “growl”
Experiment with these shapes and then feel free to come back and we can help you with the next bit
Good luck!