To catch fish, you need an Old Fishing Rod, which can be bought at the Market for 5 crowns. Its lifespan is 10, so you can only use it up to 10 times. In addition, the success rate hovers around 40-50%.
Fishing is available at the farm, where you can complete daily tasks. You can teleport to the farm by clicking the "Kingdom" button in the upper right corner of the screen and then "Farm".
The farm has a limited School of Rare Fish every day, which can only be used once a day.
How to catch fish
Buy an Old Fishing Rod at the Market.
Open your bag and select a rod. Then press the "Fish" button.
The game will automatically locate the School of Rare Fish (if available). As soon as you get to the location, a pop-up window will appear. To start fishing, click the "Use" button.
You can fish up to 10 times depending on the durability of your rod.