You will get your first bamboo fishing pole for free from Willy (it costs 500gold at his beach shop).
How to fish:
Face water with the pole selected, hold
to start casting and then release at desired distance. When you get something to bite press
to start the mini-game.
The goal here is to keep the fish inside the green bar. You control it by tapping or holding
. Same as with farming and mining etc, this is a skill you level up.
Be aware some fish are harder then others to catch.
This achievement counts everything you pull up as fish, even driftwood (which is what made it pop for me).