Before you smash the ingots, but after you place them on the anvil, press "Z". This will bring up a wheel menu in which you click on what type of weapon you want to make. It has a shield icon, but you cannot click on that now because it will be implemented in a future update. There is also GIANT text on the wall next to the furnace telling you to do this and to NOT freak out. But I guess if you didn't see that you didn't see that.