Who puts the ball into a scrum in rugby?

This is a Scrum or Scrummage. The scrum half (number nine in Union) throws the ball in to the tunnel formed by the two front rows. To do this the Scrum Half stands adjacent to their open side prop forward. This is the one who has one side of his head (the left side) not in contact with the opposing prop forward. The ball must pass the feet of the open side prop before the ball can be hooked back by a front row player. No hand may touch the ball . The 2 sides endeavour to push the other off the ball so that they can win possession

To learn more about Rugby rules look on the IRB website - it has a wealth of information.