Type "iron deposit" in the search box to the left - it will give you a list of the zones where iron can be found, with maps of the known appearances of deposits in each area. Very helpful!
Iron is right on the edge of what you can probably handle at level 21. Duskwood, if you stick very close to the mountains you can avoid most of the mobs, and the Wetlands if you can sneak into Angerfang's camp or around Raptor Ridge are probably the easiest. If you really want to level up your blacksmithing before leveling up you character, you will probably need a buddy to cover you while you mine.
As LockeColeMA said, silver is a rare spawn of tin and gold a rare spawn of iron - you can't reliably farm either. However, if you make circuits of Loch Modan, for example, mining every tin mine you see, they will respawn before you complete the circuit and you will occasionally find silver.