first when u just get the skill ( this could be a long term project) level it up to about 40, once there go to the pond right in front of the wailing caverns. fish there and sell the deviate fish ( they are worth a bit i havent fished there in a long time so check the AH for median prices) once you have leveld your skill to about 150 buy some bright baubles and head to tanaris. go to the dock at steamwheedle port and cast there. during the winter there is a chance to catch " winter squid" have them cooked to make " grilled squid" these add a 10 agility buff that lasts for 15 min. a stack of 20 goes for around 2-3g.
also, you can sell firefin snapper for about the same.
there are clams that you can catchby fishing or aquire by killing the nearby turtles. the " big mouth clams" dont sell for much but there is a chance that there is a pearl inside. i have caught over 30 clams with black pearls about 15 with iridescant and 3 with golden pearls which go for 20g each ( about ).i also had enough luck to catch a clam with a golden and an iridescent in it.
but the one thing i have made the most cash off of are the stonescale eels. these you can sell to an alchemist or raid guild for quite a bit. i sell stacks of 20 for 10g. i did this so often that i bought my mount at lvl 30. i had to wait 10 lvls to use it but i didnt have to scramble for cash once i got there. they arent a rare drop and they are useful
i hope you find these fishing tips to your liking Happy fishing!!