In order to fish, you have to put on all parts of the tackle: e.g. a rood, hook, line, reel, and bait for the spin fishing technique.
Not all rods work at every fishery. You can use only spin fishing and sea rods at some of the fisheries.
If a given fish doesn’t bite the bait, you have to buy another type of bait. To see which fish bites which bait, go to the profile of the fish (available, for example, while entering a fishery).
If your line breaks and the fish gets away, you’ll see a window, which lets you decide whether you want to try to catch the same fish again for 3 banknotes.