If you want to power smelters to melt down your copper and iron ores, you will need to make some coal using a charcoal kiln. That said, here's everything you will need and how to do it.
How to get coal in Valheim
To make coal using a charcoal kiln, you will need to first make and place a charcoal kiln with 20 stone and five surlting cores with a wood hammer. To build a charcoal kiln using a hammer, right-click the hammer and then find the crafting tab; once built, place it outside on the ground and then and get any wood type and place it inside by hitting e.
As a side note, make sure to have a workbench built, and you are placing the charcoal kiln within the building circle, or it will not place. Afterward, at a rate of 1 coal per 15 seconds, you will make coal using a charcoal kiln this way.
After you get some coal, you can then use it to make a forge and power your smelters so that you can make the following items below.
You can also find coal occasionally in chests and in the swamp biome and inside heaths; along with this, you can also make coal early on by burning raw meats and neck tails in the cooking station. Surtling enemies that spawn in the swamp sometimes will drop coal as well, but these are dangerous enemies so you should by no means use them as a reliable source of coal.
Related: How to build a forge in Valheim