so one of my serfs discovered metal, it says i need a mine and some kind of machine, i am wondering where i build that machine, does it count as a structure or is it like artifact that need a slot to create.

It's a machine.
You need to build a Workshop.
Then click on any free slot and build it.

Do I have to let my serf do the "geology" search on a mountain to discover metal? or does it work on the town cell also?

let your serfs search the tiles around your city... doesn't have to be a mountain tile.

oh and don't forget to send a serf as a miner to the mine once you've built one.

Thanks Fens.

My serf is Geologing plains as we speak.