how to drill through two different materials
What you need to use is a fastener called a "toggle bolt". They come in small sizes (1/8 inch, screw diameter) up to 1/4 inch. Depending on the weight of your item the larger the toggle the more it can support. A toggle bolt is a threaded machine screw that uses a pair of spring loaded wings (anchors) as a nut. Mark the location for the hole and use a slot head screw driver to bore through the dry wall. I'm guessing that the metal you encountered is most likely the lathing for the plaster (or stucco?) wall. Either way,try to make an oversized opening into the bay of the wall. Put the toggle screw through the item and thread toggle wings onto threads. Push the two wings together and wiggle them into the wall opening. The wings will expand behind the drywall. All that's left is to tighten the screwhead. Finito.