The flow of the game goes from biome to biome. Each time you enter a new one, you take/process whatever resources it just unlocked for you to upgrade your weapons/armor/structures, then you beat the boss to move onto the next one (the bosses usually drop an item that you'll need to progress). For the black forest, you need to mine copper and tin to create bronze which is needed to upgrade your weapons. You'll also gain access to fine wood when you make the bronze axe, which is needed to create some weapons and pretty much all of the furniture pieces. You can of course enter any of the biomes in any order you want but if you get too far ahead without upgrading armor and weapons, you'll die easily.