Fish can be caught in the sea, rivers and ponds once you have a fishing rod.
This table lists all of the different fish, along with their sale price in bells. Some fish are worth a lot more than others, especially sharks which can be identified by their fins.
Prices will vary based on a few factors:
While fishing you will sometimes fish up junk. Junk acts just like fish (swims around, bites at your lure), but costs money to dispose of. Each piece of junk costs 80 bells to recycle. The various pieces of junk are: A boot, A can, A tire
You will also find shells scattered along the beach. Early in the game, these are a quick source of bells. Later, their value is generally too low to bother collecting.