You're not going to be very successful if you don't have the money to upgrade your weapons, tools, build barns, and complete the house renovation necessary for marriage. There are several different ways you can earn Gold.
The items you sell have a Level associated with them. The price you receive for selling varies depending on the level.
Profit = Level x Base Price
For example, you found 2 pieces of Iron Ore while mining. When you go into your Rucksack and select them, you notice one is at Level 1 while the other is at Level 12. When you sell your Level 1 Iron Ore you'll only receive 50 G, but your Level 12 Iron Ore will net you 600 G!
Growing crops is the most basic things to do in any Harvest Moon game. Each season you'll have a new set of plants you can harvest.
There are several different areas where you can catch fish. You can cast away in the streams, in the ocean, and even inside the monster filled caves!
In order to upgrade your own weapons and tools, you'll need to find the ore hidden in rocks inside the caves. Sometimes you'll even find a jewel you can ship for a lot of money.
Some of the monsters you catch can produce their own goods for you to ship. There aren't very many different types, and not all creatures of the same family will produce items to sell.
All of the items available for Rosetta to pick up are listed in the shipped items list in your menu screen. You can see how much G you have earned for each type of item.
You can earn more money if you take your animal produce and transform them into higher selling items by using the Maker machines inside your house.
Most things you want to sell can be put into your Shipping Bin on your farm. The other option is to sell items directly to the shops in town. All of the shops will pay the same price so you could sell your swords at the Bar if you wanted to!
The profit you receive from selling your items to shops will immediately added to your funds. If you send it to your Shipping Bin instead you'll have to wait until Rosetta comes to pick up your good at 5:00 pm to receive your money.
The disadvantage to selling items to shops instead of via your shipping bin is the items will will not appear on your Shipped Items list.