Rotate an ingame 2D object via script just like the rotate tool

I have a 2D sprite which I want to rotate just like the rotate tool in the editor. The following script I have rotates the object in all axis I just want rotation in z axis just like the rotate tool with mouse .I am totally new to programming and really need some help.

 public float RotateSpeed = 15f;
 void OnMouseDrag()
  {
      transform.rotation = Quaternion.AngleAxis(  Input.GetAxis("Mouse Y") * RotateSpeed, Camera.main.transform.right) * transform.rotation;
  }
 

}