Money (Gold coins) are obtained by killing mobs. When a mob dies, it will drop white coins. Not all mobs drop coins after death (bosses). Coins can be obtained by doing the optional quests. The list of quests is displayed after pressing 'G'.
Optional Quests started at listed people at Port Juno :
Harold - catching fish,
Frank - killing zombies,
Gardener - obtaining bone meal,
Miller - killing outlaws,
XYZ - killing creepers.
Each of the quests is rewarded witha certain amount of money and XP popints. Coins can be used to pay for teleportation, buying armour, weapons, ores, mining tools, horses, ingredients for making potions and the potions themselves.
In the HUD
Money appears in the HUD, or Heads Up Display, beside the Health Bar as a white circle with a number beside it. This number represents the amount of money you currently have.
[Picture of the money icon in the HUD]