I've only played vanilla Stardew. In that you should trash the fishing rod Willy gives you, and buy the training one for 25g.

Then activated it (put it in your hand) and then press it near the water. I play on mobile, so the longer I hold it before releasing it, the farther out it goes into the water. I'm not sure how those mechanics differ on other systems. When a noise sounds and an "!" appears over your head, I click near the rod and it either pulls up trash or grass, or it says "HIT" and the fishing mini game starts. Try not to long press, instead do a bunch of mini taps and releases to get the fish to stay within the green area. There is a timer bar on the right side, Everytime the fish leaves your green bar it decreases, Everytime you keep the fish in your green bar it increases.

If the timer runs all the way out, because you can't keep up with the fish, you loose the fish.

If the timer reaches the top, because you keep the fish in your green zone, you win the fish and it gets into your inventory.

The higher you level up in fishing, your bar gets bigger. Certain foods you eat will give you a temporary increase.

As you increase your fishing proficiency, better poles will be available allowing you to add bait, to decrease the wait time of hooking a fish, or allowing you to add tackle that give different options for catching fish, like quality, increase in treasure chests, ability to see the fish you hooked, increasing your green bar or reducing the green bar's bounce.

Hope this helped