I set my iPhone 6 down at store and it was turned all the way off and I couldn't track it so I think somebody picked it up I wanna know how can I track a iPhone while its turned off
Simply put, you can't. You can lock it and mark it as lost on Find My iPhone. That way if/when it comes back online it will notify you and record its location.
Unfortunately if the thief knows what they are doing, I doubt you will ever see your phone again.