All mines do is increase the "shield" output of the square you mine. Shields are generic production units, used to build stuff in your cities.
Strategic Resources, on the other hand, show up on the map once you have the technology to see them. By "armor" in the Middle Ages, I assume you mean Iron. You need the ironworking tech, and then hit Ctrl+Shift+M (clear the map) and look for the iron symbol. The Civilopedia will show you what it looks like. You must build a road to the resource in order to use it.
I'm not sure what you mean about attacking from galleys. If you mean "how does one invade another island" then here's what you do:
You know how to put units on the galley, right? Well, sail it over to the spot you want to drop your troops off. The square must be empty of AI units. Then you can either press the movement key that would bump the galley into the land square and you will get a unloading popup, or you can just click on the galley, activate the units in it, and move them onto the square like you normally would. You can't attack until the next turn. The exception to this, late in the game, are marines, who can attack units and cities directly from ships.
-Arrian
