Having a proper grip on the golf club is the most important fundamental in golf. An incorrect grip will not only result in poor golf shots, but the longer one goes on golfing without having a proper grip, the more natural that incorrect grip will feel and the harder it will be to feel comfortable with a correct grip.
There are two different ways to grip the club during a round of golf. The putting grip is the first, and the second is for any shot that is not a putt. The putter can be gripped a variety of ways, whereas the other grip should be pretty much the same for everyone. If one grips the club in the 'conventional fashion' when putting (as opposed to a 'claw grip', 'belly-putter grip', or a garden variety of other options), both thumbs should be set flat, vertically, and in the middle of the grip directly in front of the golfer's body, with the back of the left hand and the right palm facing the target line.
Whether the left hand is set lower on the putter grip or the right hand lower is up to the individual. Keep in mind, however, that when PGA professionals are asked the question, "what is the one thing you would have done differently in your career", the majority of them say they wished they would have started putting with their left hand low (or 'cross-handed') rather than their right hand.
The reason why cross-handed putting can be beneficial is because it's easier to keep the left wrist firm during a putting stroke. With a right hand lower grip on the putter there is a tendency to break the wrists a bit at impact, thereby making distance control more difficult to judge and accuracy less than desirable. Additionally, cross-handed putting grips set the shoulders a bit more level at address, which encourages a more even stroke on the ball and through the impact zone.
A beginning golfer should experiment with both the right hand lower and the left hand lower method to determine what works best for them. It is advisable to avoid the unconventional type of putter grips unless nothing else seems to be working.
For every shot in golf that is not a putt, there is a standard grip the golfer should impart on the club (with a couple of subtle exceptions). The right hand will always be lower than the left for this grip. Whether a golfer chooses a baseball grip (hands separate but touching), an interlock grip (right pinky interlocked between the forefinger and middle finger of the left hand), or an overlap grip (right pinky rests above the crook between the right forefinger and middle finger) is up to the individual - with the recommendation that better golfers generally use an overlap or interlock grip. The baseball grip might be more appropriate for children (because it feels more natural to them) or seniors (because they are looking to use more hand action due to decreased shoulder turn).
The left hand should always be placed on the grip before the right hand. The left thumb should be flat along the grip, but just right of center (so that the first two knuckles of the hand are visible). Most beginners put the thumb right down the middle of the grip, but this makes it too difficult to properly rotate the clubhead as it comes in to contact with the ball (often resulting in slices). It also puts the club in a bad position at the top of the backswing which makes it difficult to return the clubhead to square when it makes contact with the ball. If the hand is rotated clockwise a bit so that the left thumb is a bit right of center, the V that is formed by the thumb and forefinger will point between the chin and the right shoulder. This will allow the club to get set in a square position at the top of the backswing, with the left thumb directly under and supporting the club, and will allow for the clubhead to naturally rotate through the ball.
After the left hand is set in a good position on the grip, the right hand should be set with the left thumb set along the lifeline of the right hand. The right hand should grip the club more in the fingers than the hand (whereas the left hand grip is more along the lifeline in the palm). Again, the V that is formed by the right forefinger and thumb should point between the chin and right shoulder, the exact same direction the V from the left hand points.
Although the thumbs and hands are in slightly different positions on the conventional grip than they are on the putting grip, the back of the left hand and the right palm should still point along the target line.
