You are most definitely not alone. So to briefly explain, you have to cast into water. When the bobber goes completely under you’ll need to left click. Assuming you timed it right the game will display the fishing UI. Your goal is to get the blue bar that is constantly depleting at the bottom all the way filled.
You do this by pressing the corresponding keys for the button prompts. You are pressing the buttons from left to right. Despite them being arrow prompts you should still be using the WASD keys. So W for up, A for left, etc. For every correct key you press the meter expands. For every incorrect press the meter will contract. A word of advice making sure you are accurate is (in my own opinion) better than going fast. For every correct button press the meter will degrade slower so it gives you more time. You should also keep in mind that different fish show up in different places and some are far harder to catch than others.
Fishing is going to be looked at in the near future because most people seem to find it difficult. Below is the link to the discord server, the people there tend to have advice for these sorts of things as they are playing too. Such as where the easier fish are at. Hope this helped.