The firepit is the basic source of light and heat for a new player. They are small, compact, and require plenty of wood to keep going. You use them to create torches and cook food.
To create a firepit, simply drop three sticks and a log and use the firestarter on them. Often several attempts are required before successfully lighting a fire. If you wish, you can also add a piece of paper as tinder, to improve your chances of initially lighting the firepit. The paper is consumed in the process of course. Firepits do not spread fire to nearby blocks. The firepit requires plenty of wood, and thus will be the secondary reason you want to gather logs(the primary reason being to make charcoal in a charcoal pit). Right click to open the firepit interface and add logs to the fuel slots, then restart the fire using your firestarter.
It is possible to smelt low tier ores into metal using a ceramic mold, however, this is quite pointless. The resulting metal cannot be used to make tools, and cannot easily be combined with copper to produce bronze alloys. Cassiterite, sphalerite, and bismuthinite are the only three metals that firepits can smelt, since it cannot get hot enough to smelt any other ore. To learn how to create copper and bronze, see our article on pit kilns.
Firepit GUI, from left to right:
Left Squares: Fuel slots. Insert logs into the top left slot (or simply shift-click the logs in your inventory) to keep the fire going. Coal cannot be used.
Middle bottom: "Output" squares. When smelting ores, it is essential to place a ceramic mold, or tool mold, in the "output" square, or the metal will be lost when it liquefies.