A hooked ball usually travels low and curves to the left for right-handed golfers. The ball rolls farther than the straight shot. For this reason, the draw, a miniature hook, is one of the strongest shots in the game. Common Causes: Improper 'strong' grip (common), closed stance, horizontal swing plane, the club points right of the target at the top of the backswing, poor shoulder alignment.
An 'inside to outside' swing arc puts counter-clockwise sidespin on the ball, causing it to hook to the left.
Hooks are caused by too, 'strong' a grip. The right hand is too far under the club grip at address. At impact, the hands rotate to the left and 'close' the clubface. Weaken your grip by moving the right hand higher on the grip. See the proper grip section for more detail.
Check your set up. The shoulders and hips should be parallel to target line. Swing more upright. A flat 'horizontal' swing can cause a ball to hook. The left arm should point down at the ball at the top of the backwing. Club should point down target at the top.