Refer to the picture below as I explain...
1. Equip your pickaxe in your shield slot.
2. Find a pile of ore on the mine floor.
3. Click on a pile of ore.
4. The BattleText box will tell you your chance to mine them, and how many bars of metal you have mined.
If this did not work...Make sure that you have the mining skill. Also, make sure that you had no other items equipped while you tried to mine.
If you plan on selling your ore, I would suggest selling about 15 stacks of 20. The reason I say this is because most players will only buy 15 stacks or more at once. (15 stacks of iron is worth 2500-3000 gold).
Another thing... At 15 mining skill, you can mine 2 bars, 25-3, 35-4, 45-5, 55-6, 65-7, 75-8, 85-9, 95-10).
Also, the better the metal you mine, the harder it is to mine.