Purchase Upgrades at the company store!
Upgrade your pickaxe to mine deeper and faster. The deeper you go in the mine, the better ore you will find!
You can also buy Rope to help for climbing down and back up tunnel shafts. It's useful for quick access, and also if you get stuck.
The bombs are a powerful way to deal with Monsters. Bombs are fairly expensive though, so you may need to learn magic to fight off everything that lurks in the mine.
You can also sell the ore and items you find to the company store for profit. Items may be useful later in the game, but many of them can be sold for a decent amount of coins as well. Ore is worth more the deeper you go in the mines, so to get rich you'll need to keep digging.
Certain items can't be sold to the Company Store, but these are mostly special items that are used in the game story progression.