One of the most common faults I see with players is starting the downswing with their arms and shoulders rather than their lower body. This is often a cause and effect problem with the cause being an out of position right knee. Let me explain.
Regardless of your skill level it is imperative that you coil into the right knee when making your backswing. The right knee serves as the anchor which provides the resistance against the coiling of your upper body in the backswing. If the right knee collapses or straightens the resistance is lost
The two errors are locking the right knee where the left knee moves out toward the ball, and buckling the right knee where the player's weight moves to the outside of the right foot. Both faults lead to a poor turn forcing the player to start down with the upper body.
To correct this focus on flexing your right knee at address then retain the flex all the way to the top of your backswing. A sign that you've done it correctly is feeling some tension inside your right thigh as your trunk coils against it.
A good drill is to turn your right foot in at address about fifteen degrees then make some swings. You'll quickly feel the resistance in your right knee against the winding of your upper body. This in turn creates tremendous power delivered to the club head as your legs and hips unwind in the downswing.