Watch the Open A Pedal Point online guitar lesson by Matthieu Brandt from Chord Cookbook
The open A string is one of the easiest pedal points to use. One of the reasons for this is the easy access to the E and D strings, which are the roots for the IV and V chord in both A major and A minor. And there are lots of chord shapes you can move up and down the neck on the middle three strings (the 2nd, 3rd and 4th). You'll be creating all kinds of harmonic motion on top of a sustaining bass note A.