Basically, the heavier you are, the better. The extra weight penalizes you in long, extended fights, but wounds penalize you for any fight.
If you have absolutely no metal : go leather. Bone and scales are good if you don't have a meat industry. Be aware that any metal weapon will cut through any of the organic armors tho, but if you can downgrade a crippling wound into a recoverable wound, this is all the better. Plus : training in armor increases armor experience, which allows you to fight longer in extended fights.
If you have metal, litterally any metal is better. Iron and bronze are almost equal, so if you have a lot of copper, buy or mine tin. Steel is not a hard requirement ; if you can outfit heavy copper infantry, provided they are well trained, you should be able to win most of your engagements.
Keep in mind that breastplates only protect your body, and chainmail protect body, upper legs and upper arms, altho to a lesser extent. It is best to have both.
So I think the best advice is, first work with what you have, then slowly upgrade your gear. Leather is cheap and easy to come by even without trade, and can be made into almost any protection piece, including shields (wood become a limiting ressource midgame, you might want to save it if you can) so it is a good starting point. Then make copper sets, forge every copper bar you can into bronze and upgrade. Stopping at bronze is enough for most ennemies the game will throw at you.