The standard sitting pose for playing acoustic guitar has the guitar resting on your right leg. Your right arm can drape over the lower bout of the guitar, with the instrument fitting snugly in the crook of your arm. That should leave your right hand dangling somewhere between the bridge and the soundhole, or right at the soundhole's edge, when you go to strum or pick. If you’re planning to stand up, you might want to have a strap button attached to the heel of the neck to help center the guitar’s body on you and ensure a stable placement of the instrument.
Keep your left-hand thumb along the back of the neck, wherever it feels most comfortable. It can creep up over the top, past the fingerboard, as long as it’s not actually touching the low string and keeping it from ringing out when you strum. Be sure to keep your fingers arched, with each joint bent; if they flatten out across the strings they will also flatten the sound when you strum.
HOLDING THE PICK
There are a number of ways to grip the pick. Here’s one standard way, which you can change to suit what’s most comfortable to you as you get going.
Start with your four fingers curled loosely together, as if you were holding a broom handle. Place the wide end of the pick on the first joint of your index finger. Now bring your thumb down to hold the pick in place. If you keep your fingers curled like this—not tightly, but in a little group—they’ll be out of the way as you strum the strings.
The thickness (or gauge) of the pick you use affects your sound, and the way you grip it affects your ability to get a good strumming sound on both downstrokes (toward the floor) and upstrokes (toward the ceiling). Try a standard-sized, medium-gauge pick to begin with, and hold it with a moderate grip: tight enough that it doesn't slip out of your hands, but loose enough that you can wobble the tip of the pick back and forth with your other hand.
Excerpted from The Acoustic Guitar Method