How to catch fish

Fishing Classes
To get past the level 1 hump, Sims can take a fishing class at the grocery store in town. This usually costs $400. It's worth it, because your Sim will make that first level much faster than by fishing normally.
How to Learn Fishing
Once your Sim casts their line, they will immediately begin learning how to fish. The fishing skill will be learned within a few hours, and your Sim will finally have the skill appear in the skill journal. You can then view your progress toward the fishing challenges, view a list of fish you've caught or view the bait (once your Sim has learned a bait).
You'll catch minnows and goldfish until you can move up to a better fishing spot. That said, even higher level fish don't start providing a lot of money until you can catch higher quality versions of them by using bait.
To actually catch fish, you'll just need to go to one of the aforementioned locations and click the water. Your Sim will begin fishing. You can make decent money right when you start. Nothing like having a job, but enough for your Sims to live on. The goal is to keep practicing so that your skill gets better and better. Eventually, you'll be allowed to use bait.
Mastering the art of catching fish will not only earn you tons of Simoleons, it also synergizes well with The Sims 3's Gardening and Cooking Skills.