You need surtling cores, which come from burial sites in the black forest or surtlings (the latter you'll encounter later), and copper (from nodes in the black forest and the antler pickaxe) to make charcoal and ingots for a forge. The burial chambers look like doorways with big stones framing them, sometimes depressed in the ground. Be careful, there are troll caves too, they're fairly similar but much bigger.

You'll be fighting skeletons in the burial chambers, bring a club type weapon, and a shield if you're not confident about fighting lots of them.