With the Genshin Impact 2.1 update, Travelers can now collect various types of fish all over Teyvat. To begin your journey as a respected angler, you'll first need to find the Exploding Population world quest in your journal and toggle it as active.
Speak with Katheryne at the Mondstadt Adventurer's Guild desk to find out about the recent boom in the fish population. Then, go see Nantuck the fisherman over at Cider Lake, just outside of the Mondstadt city gates.
After this first interaction with Nantuck, he'll request that you speak with him again at a different Cider Lake location. Talking to him here automatically unlocks the fishing pole, which you can use at any Fishing Point in Teyvat.
Basic Fishing Guide
To identify a Fishing Point, look for ripples in bodies of water. You can also discern these areas by the colorful fish swimming around. Don't get too close to the fish, since standing over the water or performing combat actions will scare them off. Instead, interact with the Fishing tab that appears when you get just close enough to that specific body of water.
Hold the Cast Rod button down to aim and then cast your line out into the water. Aim close enough for the fish to see your hook, but try not to aim your line directly at them, as doing so will frighten the fish. When you're ready to release the line, let go of the button.
When a fish is interested, it might bump into the hook a few times before biting. Time your reaction just right so you can press the Raise Hook button as soon as possible.
Raising your hook will cause the fish to struggle. Like most other fishing mini-games, you'll need to balance your line tension by keeping within the yellow bar and allowing the meter to fill up. Once the meter fills, you'll be able to reel your catch in.