Start out mining copper...Mulgore for the Horde, Loch Modan for the Alliance are two good spots. Talk to the Mining Trainer and make sure you learn how to smelt the ore you mine into bars to sell on the Auction House. Once you get high enough in mining and smelting copper, go searching for Tin in slightly higher level areas. Mountain areas are always good to mine, cause the mineral nodes don't spawn in open fields. It is a very time consuming and slow profession to level up, but it does end up paying off in the long run. For example, a stack of 20 Copper Bars go for anywhere from 50 silver to 5 gold. Just gotta play the market.