To fish in Animal Crossing: Wild World, you need a fishing rod. Then, you need to go a body of water and look for a fish-shaped shadow. That is a fish. To catch the fish, you need to cast your fishing line so that the bobber is in front of the fish. If the fish sees it, the fish will swim toward the bobber and nibble on it. When the fish bites, the bobber will plop under water. That is when you should pull up the rod. If successful, you will catch the fish!
Getting a Rod
You can purchase a fishing rod in Tom Nook's store. Another way to get the fishing rod is to talk to Tortimer during the Fishing Tourney without a fishing rod in your pocket. Tortimer will give you a free fishing rod so that you can participate in the Fishing Tourney.
Fish live in water. Your town probably has a beach, a river, a waterfall, and some ponds. Different types of fish are available in the different bodies of water. If a fish is in the water, you will be able to see it as a dark, fish-shaped shadow that is swimming around.
Casting your fishing line
To cast your fishing line, look in your pockets and drag the fishing rod to the picture of your person. This will equip the fishing rod.
When you press the A button while holding the rod, you will cast the fishing line in front of you, a fixed distance away. If you are not standing close enough to the water for the bobber to land in it, your person will frown and pull the fishing line back. But if you aimed correctly, the bobber will land in the water. You can also tap a spot with your stylus where you want the bobber to fall.
Getting the fish to bite
When fishing, you need to cast your line in such a way that the bobber lands on the water in a spot where the fish will see it. The bobber needs to land fairly close to the fish, and somewhere close to the fish's face (the rounded part is its face, and the pointy part is the tail). If the fish can see the bobber, it will swim up to it and start nibbling on it. You might have to try multiple times before you cast the bobber somewhere where the fish can see it.
What to do when the fish bites
When the fish takes the bait, there will be a splash of water around the bobber, and you will hear a distinctive "plop" sound. When this happens, tap the screen with your stylus or press A as quickly as you can to pull up the fishing line. If you pull it up in time, the you will catch the fish! However, if you're not quick enough, the fish might get away. Also, if you pull up the fishing line before the fish takes the bait, the fish will swim away and disappear.