In Standard tuning, the 6th string root note and on the 5th and 4th string move 2 frets over. For instance the first 5 frets on the 6th string power chords would look like this.
Strng4---2--3--4--5--6--7 (Root Note)
Strng5---2--3--4--5--6--7 (5th of the scale)
Strng6---0--1--2--3--4--5 (Root Notes)
.............E...F..F#..G..G#..A
.....................Gb......Ab
The 5th String works pretty much the same way. Don't play the 6th string (you can mute it with the tip of your index finger).
Drop D tuning, where the 6th string is tuned down 2 half tones (2 frets) from E to D is interesting because your power chords are all on the same fret. The E is the second fret the F the 3rd and so on. Single finger barred power chords. You'll that find many of your heavy metal bands that thrive on power chords do this.
But the KEY is to know the notes on the 6th and 5th strings. This will also become invaluable when learning barre chords.
Hope this was of some help.
Michael