New Player Fishing Guide

To fish you need a rod, bait, and a body of water with fish in it. 
Rods
Rods will determine the size of fish you can catch. At their most basic there's rods for stackable fish(Halcyon is the best here) and rods for unstackable fish(Composite rod). Halcyon and composite rods can carry you all the way to 100 fishing. You'll have an easier time skilling from 75ish onwards with a lu shangs, but that's a long quest and many will choose to skip it and go straight to Ebisu(The ultimate unbreakable rod).
Bait
Bait determines which fish will bite your line in any given pool. Our fishing pools are based off of Great Blue https://www.great-blue.jp/?lng=en. 
Skilling up
For best results while skilling up, you want to target a fish that's 7-11 levels above your current BASE skill level(Guild support and fishing gear do not effect skill up rates. It's base skill vs fish level only). As you get further away from that sweet spot the skillups slow down more and more. You can catch a fish 30 levels above you, but don't plan on skilling off it.
Why Fishing?
Fishing is a great way to make some money direct to npc(Though some fish do do well on the AH like Moat Carps, you'll be npcing most of your catches). At 100 fishing + ebisu rod, you can expect to make at minimum 60k/hr whenever you decide you want some gil. Fishing won't make you rich, but it's consistent and doesn't require any luck.
Fishing Pools
All fish in an area have a respawn timer. If you're at the right spot with the right bait and not catching anything, chances are the fish are all fished out. Pools replenish regularly and quantity of fish varies based on where and what you're fishing. 
How to Fish
Get the rod and bait and go to your desired fishing area, type /fish to cast your line. play the arrow mini-game by hitting the arrows and when the fish's stamina is depleted press enter or confirm to reel it in. Pay attention to the chat log when you hook a fish, If something clamps onto your line ferociously it means you hooked a monster, press escape or cancel to reel your line in and cast again. Items, fish, and monsters all have unique lines of chat and once you get a feel for an area you'll know what you have hooked just based on the stamina bar. 
Fishing Gear
If you want to fish, you need the gear. It should be your first priority to sell your moat carps on the AH and buy the whole set when you can. Followed by composite and halcyon rods. 
Getting started
*This is in a very very unfinished state, just copy and pasting some of my notes over for now.*
Farm a Clothespole from goblin fishers to get started
Willow rod works for moat carps as well
0-11 Moat Carp(11 cap), Can use one of the rods suggested above or halcyon rod. Insect paste is best but little worms will get you started if you need cheaper bait. Fish in Knightwell in West Ronfaure or Lake Tepokalipuka in East Sarutabaruta. Both have the same moat bite rates. 
11-14 Quus(19 cap)" Sabiki Rig, fish in pool by giant clam in korolokka tunnel
14-18 Nebimonite(27 cap) SSG pond, shrimp lure. Get advanced support if coming here at 14, left at 18 for catfish
18-23 Giant Catfish(31 cap) Port Sandoria, Composite Rod + Minnow. Started at 18 with adv support
Alternate to Catfish: Dark Bass (33 cap) Jugner forest Lake Mechieume, Composite Rod + Minnow
23-29 Alabaligi(37 cap) Bhaflau Thickets Composite + Fly Lure. Started here at 23, Stopped getting skill too low messages at 25, low npc price(26g each) use for skill ups and leave asap
29-33 Coral Butterfly(40 cap) Kahzam dock, Halcyon or composite, Worm Lure. Started here at 29
33-42 Icefish(49 cap) Beaucedine Pond right by the outpost, Halcyon and sabiki rig, Started at 33 with advanced support and it was a little low still but worth it for the money
42-48 Bluetail(cap 55) Buburimu, Halcyon and Minnow, Started at 42 with adv support
48-52 Elshimo Newt(60 cap) Yhaoter Jungel underground pool at E-9. Composite or Halcyon with Frog Lure. This one is awkward, these don't stack so running back to sell gets tiring. Some people might opt to just fish with a full inventory here. Even if you're full inventory you'll still get skillups
Alternate: Bhefel Marlins (61 cap) Mhaura <> Selbina boat, Composite and Bluetail slices. If you want Ebisu rod you're going to need to ride this boat eventually for Sand Charm quest anyway, so might as well do two birds with one stone. A bunch of stuff likes bluetail slices so this is the slower skill up route for sure, but they AH well enough. 
52- Crescent Fish (69 cap) East Sarutabaruta in Lake Tepokalipuka. Halcyon Rod and Fly lure. This is the first fish where a small fish pool will give you trouble. If there's a lot of people in Saruta, you can make the long trip to Dragon's Aery where the pool is much larger. At 52 skill I'm getting .1-.2 skill per stack of fish and they npc for 400g each. It's worth leaving Newts early for these especially if you're looking for some extra gil. Stopped getting low skill messages at 55 with advanced support, 57 without.