Fishing is one of the many different activities in the Animal Crossing Series. It is seen in all of the games. Fishing is normally looked at as one of the more basic activities in the game, which in a way, it is. However, there are many different extensions and things to know that don't affect you if you are just a casual player. Fishing is the only method to recieve fish unless, you buy from other villagers during the Flea Market.
 The Basics
 Fishing Rod
The Fishing Rod is the only item you need to fish (that and yourself!) These can be bought at Tom Nook's Store (all four of the installments sell them), so be sure to pick one of up if you want to participate in this fun game in Animal Crossing! Like all other tools, they are only 500 Bells, so all you need to get it 5 Fruits or 1 Foreign Fruit and sell them to get enough money.
 Fish, Trash and Keys
In the river and ocean, you can uncover three different types of objects: Fish, Trash, and House Keys that your neighbors have lost. Fish are the most common, with trash being the second-most popular. Keys can only be found when a neighbor tells your character that they have lost their key, and you offer to look for it. Keys and trash do not sell for anything, but fish do, which brings me to my next point.
Fish are a great way to make money, or to decorate your house, or to fill up the Museum. They come in many different shapes and sizes, and you will find many different types of fish throughout the series, appearing in different seasons. There are the common species that everyone knows about, and the rare species that are rarely seen.
Trash varies from boots to tires and possibly many other things. These are truly useless, as they cannot be sold, only thrown away by Tom Nook. If one is caught, throwing it back is your best bet to get more Bells, as that spot in your inventory can now be used for other, more important things, such as keys and fish.
Keys are a little different from both trash and fish. There is only one at a time, and it will belong to one of your neighbors who is locked out of their house. You have to talk to them when they look stressed, and if they tell you they dropped their key near the river, you know where to look. Pull out a trusty fishing rod, and you'll get that key in no time!
Whenever you're fishing, you won't ACTUALLY see the fish (unless it's a shark, then you will see his fin), but you will see a shadow in the shape of a fish (even if you are going to get trash or a key). Your goal is to make your Fishing Rod follow that shadow, and hope that shadow latches on to your bait. The best way to do it is to throw your bait near the fish, but not too close, or it will run away.
Bigger shadows usually represent a bigger fish, but they will also represent trash(you'll never be able to differentiate one from another). Smaller fish do the same thing, but with smaller fish. If you are looking for a certain type of fish, pay attention to the different shadows that fish have!
 Advanced Fishing Tips
Never Run when near a River/Ocean. This is only applicable if you want a fish or other item. There's a very good chance you'll scare it away if you're going too fast, so make sure you're always walking or creeping. Especially if you are going for a shark or bigger fish, as they seem to run faster strangely.
Bigger Fish does not mean more Money. This may seem strange, but it really isn't when you think about. Tom Nook gives you money by species, not by how big the fish is. Make sure that you understand this, and go for all types of fish that there are in the waterway you are using. Otherwise, you are just wasting possible Bells. We wouldn't want that, now would we?
Fish never bite more then five times when going for your lure. Oddly, fish will vanish after the fifth attempt at your bob. Be sure to take advantage of this; if a fish does not grab on until the fifth attempt, squeeze that button!