Go to a welding supply store and pick up some mild steel rod. I think you should read some on welding before you start. Brazing with brazing rod and welding are two different things. Brazing is more like "gluing" with the torch by melting the brass over two or more pieces of metal to join them. Welding is actually melting the pieces of metal together making one piece.
You need a very small tip to weld sheet metal.