First off, did you get the Bee Larve?
If not, do that, look it up or buy the guide on how to do this.
1. Walk anywhere near water (duh)
2. Bait hook with Larve
3. Get the pole out and cast with the Wiimote
4. When the bobber is straight up, and then starts to get pulled under, YANK UP, and it will say something on the screen.
5. Start to pull the controller up and away, like you would while fishing.
6. The controller will vibrate, and if it vibrates too fast, let a little slack out by moving the wiimore closer, or the fish will break away with the line.
TIP: Look in the water you are fishing, you can normally see fish swimming.