If you're a starting miner here's some tips where to get some ores, how much you should buy or sell them for, and how some are used.
Copper Copper is found throughout Elywnn forest and Dun Morogh, especially in the mines. Copper is the starting base of mining and can be used for many things, even some advanced items such as the ironforge breastplate, but more importantly is necessary to make bronze. I'd sell 20 bars with a bid of 6 and a buyout of 8.
Tin I find most of my tin in Duskwood or Westfall. You can also find tin in Loch Modan. In Westfall the gold coast quarry and the dead mines have about a 50% chance of having copper, a 45% chance of tin, and a 5% chance of silver. Also, you have a 50% chance for tin, 20% chance for iron, 20% chance for copper, and about 10% chance for silver in Duskwood. Tin cannot make any items and is only used for making bronze. I don't think tin is worth selling, I think you hould smelt it into bronze, then sell it.
Bronze Bronze cannot be mined and is only used for making items. When you smelt 1 copper bar and 1 tin bar, you get 2 bronze bars. Now, you can get A LOT of money from selling bronze, to venders and other players, but not raw material to venders. For instance, there is a blueprint that makes rough bronze leggings, and the leggings sell for 10 silver to venders. It requires 6 bronze to make: that's 3 tin and 3 copper, so if you find 1 copper vein and 1 tin vein, assuming you get 3 from both, that's 10 silver right there. I'd sell bronze 20 bars for a bid of 22 and buyout of 30 or so.
Silver A pretty much useless material as it doesn't make anything. It only adds into bronze in items that are silvered, and there are only 2 items made out of silver alone: silver keys and silver rods. Silver is in Westfall and Duskwood, and I have also found it in Hillsbrad and other places. I'd sell silver about 3-10 coins a piece
Iron This is very valuable to a lot of people and makes some good items. I find almost all of my iron in Duskwood inside the ogre cavern. It is usually on the higher grounds and is worth a lot, but you can also take a lump of coal and turn it into steel. I would sell 1 iron bar for a bid of 15 silver and buyout 20.
Gold Also pretty useless. You can make keys and rods out of it but not much more. I've never actually mined it but I do know that it is found in the Searing Gorge and all those other Mordor-ish looking places, you can get a view on it sometimes on a gryphon from Ironforge to Stormwind. I'd sell 1 bar for about 25 silver bid and 35 silver buyout.
Steel Smelt a lump of coal that you can buy from vendors for 5 silver with some iron and you've got some steel. Steel actually sells to vendors for less then iron does, even though the coal is added. Steel is used in very strong items that high level players use, and is quite expensive to make. You should sell 20 steel for about 2.5 gold or so because you might sell 20 iron for 1 gold 50 silver, plus all that coal costs 1 gold. So if you're exaulted (reputation) you could make a lot of money doing this.
Mithril This is for high level players and can be found in the Searing Gorge, Burning Steps, and all those places. Mithril sort of has a silverish yellow taint to it, and has to be used in quests that require over 200 bars, which can cost you a LOT of time. Mithril is often combined with truesilver. Mithril should be sold for about 50 silver to 1 gold a piece.
Truesilver A very hard material to find, truesilver is sort of a silver to bronze, it is just an addition that strengthens a weapon. I don't know much about this material so you should sell it however you think is right.
Thorium The most valuable ore in the game, thorium is found along with mithril in the Searing Gorge and areas like such. It glows turquoise in a way. There is a rumor I have heard that in order to wear thorium armors, you must smelt one lead ore that is found on higher level miner enemies (such as kobolds) into your armor. This could make sense, seeing as how thorium is a radioactive material.