Hold out the fishing rod.
it SHOULD be on number 1, so press 1 on your keyboard
(or just click on the thing in the bottom of your screen)

Go to a body of water
depending on your ability to move, this may be the hardest step

While near the body of water, and with fishing rod in hand

Press (or hold) M1 (The left button on your mouse... unless it's a left handed mouse, in which case it's the other one)

The bobber SHOULD land in the water, but if it didn't, don't worry, try again.

Now, we wait...

until we get to- oh ♥♥♥♥ there is an exclamation mark above your head

prepare for the FISHING MINIGAME!

When the fishing minigame starts, hold down that M1 that i was talking about earlier

When you hit one of those small white barriers with numbers, start mashing
keep mashing until it breaks

Once it breaks, keep holding M1 until you either
A) Get to the bottom
B) Get to another barrier

Once you reel in for a bit, a red bar will come down

DO NOT MAKE CONTACT WITH THE RED BAR!
It will make it so your catch flees!

Get the green bar to the bottom and you have reeled in your fish!

YOU DONE IT! YOU GOT THE FISH!

Now you can either show it off to your friends
or sell it.

You sell it by going to those weird mounds of dirt with a sign next to it
You can also buy bait there TO KEEP FISHING!