lvl 180 miningFollow

I usually go to the yeti cave in Hillsbrad Foothills when I want to mine Iron and mithril. There are usually three veins their and usually one of them is mithril. Arathi Highlands is usually good too. Generally you want to find caves to mine in. Haven't found anywhere I could say that there are alway gold so can't help you there. There are probably better places, but I haven't found them yet.
As for steel, well that's just iron bars melted with coal.

My favorite iron/mithril site is the Badlands - Lethlor Ravine and the canyons off the sides hold mithril, most of the mountains and rocks in the center have iron. There is even a forge in the center of the zone with a vendor so I can condense my load between circuits.

Steel is not mined, it is smelted from iron and coal.

Gold is a rare spawn of iron, and less often mithril. Any veins which might normally be iron may sometimes respawn as gold instead. Consequently there is no reliable way to farm for gold veins - just revisit all your iron and mithril sites, and sometimes they will be gold instead.

Arathi Highlands works really well for me. But first, i suggest hitting 40 and getting a mount, as it sky rockets the rate at which you can increase your mining (and blacksmithing for me) But in the AH's, just circle the border with your mount and you'll have iron popping all over the place =).