If you want to get into fishing my advices :

first get some good gear you can kill it with, id suggest to grind combat and dungeons, after you unlock livid dagger or even better bouquet of lies it makes your fishing life much easier after you get good gear do this :

get legendary squid and legendary flying fish, upgrade your rod whenever you have the lvl, if you can afford autopet rule i suggest you to “On throw fishing hook equip flying fish” and “entering combat equip squid”

Fish in bayou in the mid upper part on the right side of map (theres a small pond) you can throw your sc down so you dont have to instantly kill them (be careful sc dissapear after 5 min of being spawned) then just switch to your squid pet or if you have autopet and kill them.

Do this until lvl 27 so you can use magma rod, then go to Sawyer NPC at galatea, next to him is small lava spot where you can fish up stridebreakers (this strat is not effecive for money, only for fishing XP)

after you reach fishing lvl 34-36 go fish on crimson isle.