Put your first (or pointer) finger on the fifth fret of the low-E string. This is the root note--A. Lean this finger gently all the way across the fretboard as if you are playing a barre chord, but do not press down on the strings. This mutes them so that you can jam without hitting random notes.
Put your third finger on the seventh fret of your A string. This is an E note. Lean this finger back and gently touch the D string in order to be sure it is muted.
You can play this chord near the bottom of the fretboard, too. Simply strum an open A string, and place any finger 1-3 (preferably second) on the second fret of the D string. Use a combination of your thumb and third and fourth fingers to respectively mute low-E and the higher strings.