Fishing is the most important skill that you can learn in Animal Crossing Wild World. Get your hands on a fishing pole just as soon as you can. Fishing is the most profitable activity that you can do (aside from running a turnip racket, but that requires a friend which is not always available).
Next go fishing. If you are fishing for cash then you should only fish along the ocean. The fish in the ocean are worth more bells than the fish in the river or the pond. Naturally you will need to catch the river fish and pond fish if you want to fill up the museum, but when it comes to getting extra bells stick with the ocean.
The fish collect in more or less the same spots all the time so after a few minutes you will know where to go to look for fish. The bigger fish are, generally speaking, worth more bells so try to catch those. This isn't always the case so you should just catch everything you can and sell it.