How Do I Get Toilet To Stop Overflow Into Tub?

Sounds like the toilet and the tub share a common drain and there is a blockage below where the two meet. There is a possibility that you have an access below the bathroom where you can send up a plumbers snake and open the drain. If not, you can try blocking the drain and overflow in the tub and plunging the toilet.  As a last resort or maybe just to save a lot of time, call a plumber or Roto-Rooter.