How do I earn gold?

Solution 1:

1 Gold Coin is simply 100 Silver Coins. They're automatically created when you pick up 100 Silver Coins, but they can also be manually crafted.

If you're looking for good ways to farm coins, have a look at this question.

Solution 2:

One of the quickest ways to rake in the coins is farming meteor heads. You'll need to break a single shadow orb to spawn the meteor. Shadow orb harvesting is hard but you can do it much earlier with clever tunneling and a couple sticks of dynamite.

For continued quick coins an artificial meteorite biome can be used instead.

Solution 3:

I find that blood moons are excellent opportunities for making some money. Build upwards into a hill and simply stay there and let the zombies climb up to you whereby you can hit them back down until they're (un)dead. It should be upwards because too many zombies on a flat surface during blood moons can overpower you despite swinging your sword non-stop.

Similarly, goblin armies are useful for making money (albeit perhaps a bit later in the game).

However you can also follow the safer routes mentioned in the wiki. I would strongly advise you to invest in a piggy bank where you should store your coins so that you don't lose half during a battle. It costs 1 gold coin though.