How I mine for fish?

[VydarrTyr]

Buy a fishing pole, equip it, put some bait or a lure in your "pack" slot, and then go stand near a body of water. When the exclamation point appears at the top of your screen, open your main menu and click on "Fish." Then you cast, wait for the fish to bite, and play the hot/cold mini-game.

[Fensfield]

Although since the alerts can be a tad vague...

'You have caught something but cannot reel it in': Cold.
'The fish is taking line': Colder
'You take in some line': Warmer
'The fish has tired but you cannot reel it in': Try this spot again.

Also make sure you use the right bait. Their effectiveness is given in the item descriptions.