To catch a fish you need to check whether there is any bait on the hook. To do this, get the gear and click on the "Fishing reels" icon in the upper right corner. If the hook is empty, put the bait on it. The icon for "Baited hook" is next to the icon "Fishing reels". If you selected the Match type of game, you are able to change the bait clicking on the baited hook again.