When playing power chords, it is common to damp the strings slightly with the right hand. This results in a tighter, more chunky sound. Rest the heel of the hand on the strings at the bridge and then pick the strings you want to sound.
It may take a bit of practice to get the desired sound: it all depends on the exact position of the right hand and the amount of pressure used on the strings. Here is an example.
A power chord can be formed on any root note by selecting the note with the 1st finger and then adding the 3rd finger two frets up on the next (higher) string. Because the formation remains the same regardless of where on the fretboard the chord is played, you don’t need to think about the name of the higher note. As long as you know the root note, you can easily find the right chord. The following example uses various root 5 and root 6 power chords built on notes you have already learnt, along with A5 played as a root 6 chord at the 5th fret and D5 played as a root 5 chord at the 5th fret.