Position of Left Hand on the Neck
To begin with, the best way to fret a note cleanly is to find the right amount of pressure it takes to use your fingertip to make a note sound clear. This is the key to developing a good left-hand technique. Pressing too hard will cause pain.
First, let the edge of the neck of the guitar rest in the palm of your left hand. You'll notice that your thumb and fingers automatically fall to either side of the neck. Now place the left-hand thumb in the middle of the back of the neck so that there is a nice space between the neck and your palm. You should be able to pivot your whole hand on the ball of your thumb without banging into the neck.
The best practice for thumb placement is to keep your thumb on the back of the neck so that it lies between your first and second fingers, as shown in FIGURE 3-5.
When you fret a note, use the tip or pad of your finger to press the string firmly to the fingerboard, as shown in FIGURE 3-6.
If you position your left hand on the neck so you can put your thumb immediately behind the place where you're pressing the string to the fret-board (as if you're trying to pinch your thumb and finger together through the neck, as shown in FIGURE 3-7), you'll get maximal pressure on the note, and it will sound clean.
Place the tip of your first finger on the first string at the first fret (as illustrated in FIGURE 3-8). Now pluck the string with your right-hand thumb.
In the diagram, each vertical line represents a string. Each horizontal line represents a fretwire. The double line at the top represents the guitar nut. The black dot or number represents the finger you should use to stop that string at that fret — in this case, at the note F on the first string.
Press the string to the fretboard roughly in the middle of the fret (between the fretwires). If you press too close to a fretwire, the string may be muted; too far away and the string may buzz. To make sure you don't mute other strings, make a point of using your fingertips and keeping your fingers as close to perpendicular to the strings as possible by arching your wrist slightly (like a swan's neck). The classical guitar technique helps in this regard.
Though a lot of players allow their left-hand thumb to come over the top of the neck, it's hard for beginners to do. Remember, the more your hand is cramped up like this, the harder it is to play the note well (that is, without any buzzing sound), and the more your muscles may ache.
As soon as your hand or fingers get sore or start to hurt, stop! It will take a little time to build up the strength in your fingers. Playing a little bit often is better than playing a lot in one go.