The metal bars you produced smelting can be hammered into different objects during the forging phase. To forge, click on the metal bar in your inventory and then click on the anvil so a menu of options to forge comes up. Some objects will use more than one bar, and if you do not have enough, the number will in red. If your smithing level is not high enough, then the name will be on black, click on it to find out the required level. the only two types of metal you can smelt but not forge are silver and gold, which after being melted into bars, are used in crafting.