The open A string will serve as the root note for the chord, so the first note that must actually be fretted is E, which can be found on the second fret of the fourth string.
In order to create a chord with more rich harmonic quality, it is a good idea to use your ring finger to fret the A note on the second fret of the third string.
Finally, place your index finger on the first fret of the second string, which should produce a C note.
When strumming the A minor chord, you can mute the sixth string with your thumb to prevent it from ringing out