Changing the way you hold the club is one of the toughest adjustments to work on, but it's necessary if you're making this common mistake with your grip.
Take a look at this golf glove that I've drawn some lines on to illustrate the problem. Many amateur golfers hold the club with the grip laying in the palm of their gloved hand—see the red lines in the photo below. When you hold the club predominantly in your palm, you can't release it properly in the downswing. Not only does that make it much harder to square the clubface at impact, it also restricts your ability to give the shot some "pop" because you don't have any energy to release at the bottom of the swing.
A telltale sign that you're gripping the club too much in the palm is the condition of your golf glove. If it is frayed or worn in the the pad below your pinky finger, or if you tend to tear them in that spot, you need to look at how you're taking your grip.
Although this might feel really strange for you, perhaps even weak, you should grip the club more across the fingers of your gloved hand—see the black lines in the photo below. This position gives you much more freedom to hinge, unhinge and rehinge the club as you swing back and through. Your hands and wrists are able to act like a lever, and the more levers you have in your swing, the more power you can generate.
Let me leave you with another thought or two about how to hold the club. If you bend into your golf posture and let your arms hang naturally, note how your hands are oriented slightly inward in relation to your body. In other words, they don't hang straight down with the palms facing each other. This natural orientation should help you understand how they should be positioned on the club, with the left hand moving in first before the right, as I'm demonstrating below.
If you couple this gripping procedure with setting the handle more in your fingers, and less in your palms, I promise you'll have a much easier time squaring the clubface at impact for straighter shots. And, of course, the thing at the top of every golfer's wish list: more distance.