How do I make my projectile deal damage to a target it hit.

 void Rocket()
 {
     currentAmmo--;

     GameObject gameObject = Instantiate(projectile, transform) as GameObject;
     Rigidbody rb = gameObject.GetComponent<Rigidbody>();
     rb.velocity = transform.forward * projectileSpeed;
 }

 void Fire()
 {
     currentAmmo--;

     GameObject gameObject = Instantiate(projectile, transform) as GameObject;
     Rigidbody rb = gameObject.GetComponent<Rigidbody>();
     rb.velocity = transform.forward * projectileSpeed;
 }

I can't get my projectile to deal damage based on a varible in the script.