As soon as you see the bobber go under the water & hear it (you'll likely want headphones for this), you either hit A (if you're using the standard controls) OR touch the screen (if using touch controls) to reel in the fish... assuming you're fast enough.
DO NOT hit A when a fish it nibbling on the line (seeing the bobber bounce) as some fish with tease you a little before they actually bite, so wait for the bite first.
It takes some practice, especially if you're used to the console versions (with rumble support), but you'll eventually get the hang of it.