Watch the Amin & D Chords online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners
A minor (Am), being a minor chord, has that same dark, slightly subdued character that the Em chord has. If you're already comfortable with the Em, notice that if you move the two fingers used on an Em up one string each, you have the bottom end of Am. Then just add your first finger to the first fret on the 2nd string. Strum an Am from the 5th string down.
The D chord has a bright, sunny sound. As you continue learning, you'll see that it's a really easy chord to modify, too. Strum from the 4th string down.
To practice, first try the chord change shown here from Am to D. Then try interchanging these chords with chords from previous clips. Again, keep a steady beat, changing chords only when you're ready - but always in time to the beat.