The Basics of Forming a Correct Grip (for a right-handed golfer)

While holding the club off the ground, waist-high, in front of you, with your left hand, and checking to see that the clubface is square to your target-line, set the grip-handle under the heel-pad of that left hand. As you close your left hand, your left thumb should be right of center on the grip-handle (in a 1:00 position). You should see two or three knuckles on the back of your hand, which should be aimed between your target and the sky. The club should feel secure in your fingers, and you should feel the left-hand pressure of your grip in the last three fingers.

Now bring your right hand to the handshake-position. Your right palm should be parallel to the leading edge of the clubface. As you close your right-hand, your right thumb should be left of center on the grip-handle (in an 11:00 position). Your left thumb should fit comfortably in the valley between the base of your right thumb and the heel-pad on your right hand.
You should feel the right-hand pressure of your grip in the middle and ring fingers of your right hand.

Try to maintain a firm but light grip-pressure throughout your swing-"as if you're holding a baby bird," as Sam Snead once said.

Weak Grip/Strong Grip

A grip that is too weak is one in which the grip-handle of the club is placed too much in the palm of the glove-hand, rather than in the fingers.
Such a weak grip causes a slice-that big banana ball-especially with a driver and the longer irons. Your grip is too weak if you see but one knuckle (or none at all) on the back of your glove-hand (the left hand for right-handed players) when you're addressing the ball, or when the V formed by your thumb and index finger on that hand points to your chin, rather than toward your rear shoulder. By contrast, your grip is too strong if you see four knuckles on your glove-hand, and when the palm of your non-glove hand faces upward. Such a strong grip often causes a bad hook, or a shot that flies low and left. You have too strong a grip if either of the V's on your hands points outside your rear shoulder.