You need to smelt the copper into a bar first.

You need more things to unlock the forge. Search for crypts in the dark forest, there you can find red glowy surtling cores. You need five of those for the smelter and also five for the charcoal kiln. The recipes should unlock once you find these cores.

Once you have build the smelter and charcoal kiln search for tin and copper. Copper you already know how to get and tin can also be found in the black forest near water and looks almost like small rocks on the ground but a bit more shiny and silver looking.