To turn your horse around you have to?

Well there are two basic ways, the English (Full contact) way, or Neck reining (Typically used in western riding and polo) I'll use turning to the right as an example (Just reverse the aids to turn left.) Sit in the saddle and apply slight pressure to the right rein, as you do this, keep your right leg on the girth and shift your left leg a tiny bit behind the girth. The horse should arc into the turn. You may also need to shift your weight a tiny bit in the saddle. Never lean into a turn as this unbalances the horse. For neck reining you should simply apply the opposite rein to the neck, so for a right turn you place the left rein against the neck, the horse should move away from the pressure.