you can make this by a GUI.Button and hit it, also you can use clicking in the object you want to shoot, this last way you'll need to use Raycast, if you are not used to raycast this is a easyway
Ray ray = Camera.main.ScreenPointToRay(touch.position);
RaycastHit hit;
if(Physics.Raycast(ray,out hit))
{
//by this if you hit a collider of some Gameobject it will detect the collision
}