How do you produce iron?
Solution 1:
Iron is produced by Smelters from Minerals and Wood. Smelters can be built once you've discovered Metal Working. You also need to research Construction to unlock Iron.
Solution 2:
To make your first iron:
1) Research [Science] 'Mining' to unlock the 'mine' building.
2) Research [Science] 'Metal Working' to unlock the 'smelter' building.
3) Assign at least one kitten as a miner to produce minerals.
4) Assign at least one kitten as a woodcutter to produce wood (fuel).
5) Build a smelter and turn it on.
6) A smelter consumes minerals and wood and produces iron.