You click your mouse (or touch screen) when the bobber is within the green arc, pretty straightforward. That will rise your catch meter on the left, should take a few hits to fill and catch the fish.
Also, have the tutorial pic for full context. I saved it in advance, I dunno how to see it again without loading a save before fishing for the first time.
Really glad for your help, but my health is too bad. I can only use 2 fingers on the keyboard, and using a mouse is next to impossible. I have tried to use various programs that are supposed to slow down the screen, but none of them work on this game. I just am going to have to quit the game, as there is no way to proceed without catching the fish. I'm bummed because I really like the game. Again, thanks.