Once you reach level 15, you can start mining.

Go to buy a Pick Axe from the Storekeeper in Twin City (419,361), right click on the Pick Axe to equip it, ask Conductress outside Twin City (437,441) to teleport you to the mine on the top left corner of Central Plain, click on the swirling portal to enter the mine, right click on the ground to dig ores or gems. Mining does not cost health or mana.

Click on the ground to stop mining. After your inventory is full of ores, you may sell them to the merchant NPCs in a city

There are four kinds of ores: Iron, Copper, Silver and Gold. Iron is the cheapest and gold is the most expensive. Each type of the ores is classified as 10 grades. For example, Iron has 10 grades from grade 1 to grade 10, grade 1 is the cheapest, while grade 10 is the most expensive.

If you have good luck, you may get some rare gems while mining.