Strengthen your Shoulders!
Strong shoulders improves posture. Start with this Kneeling Reverse Fly exercise:
Begin on your hands and knees. Without weight, lift your right arm straight up to the side of your body. Keep your torso stable and don’t rotate your shoulders or hips. Do 10 reps, then release your right hand back to the floor. Next, lift and lower your left arm. Do 10 reps. When you have mastered correct form without weight, try lifting a small weight. This is an exercise where proper technique and form is paramount, not the amount of weight or even reps. Take your time, go slow and learn the proper movement pattern before progressing.
*Consult your physician before performing exercise.