To improve your golf swing, you need to improve your posture. If you have poor posture or slumped shoulders, it is very difficult to make a complete motion in your swing. Improving postural and core muscles will build a good physical foundation for your golf swing.

Sit upright on an exercise ball or chair, up against a wall, maintaining good posture.
Using a medicine ball or weight rotate the ball to the right touching the wall behind you, then rotate the opposite direction and touch the wall behind you on the other side.
Rotate side to side trying to touch the wall with each repetition.
Do 8 to 10 repetitions.
You can do this away from a wall. You can use a small hand weight instead of a medicine ball or no weight at all.