You may be topping the golf ball on the downswing if you are letting the club come in too steep. You will also notice steep divots as well.
How to fix it: Try to get the club coming from the inside and catch the ball on the upswing.
You may be topping the golf ball on the upswing if your swing comes more from the inside. This will lead to the club coming from behind the golf ball and catching the top of the ball on the upswing.
How to fix it: Try to feel like the club is out in front of you and a little more down.
The most common way to top the golf ball is by losing the radius in your arms when you turn back. If you let your arms fold at the top of your swing, the radius will be wrong on your downswing, allowing you to catch the top of the ball at impact.
How to fix it: Create more radius at the top of your swing.