Fishing is not only one of the most relaxing professions in WoW Classic, but also one of the best ways to make gold in WoW. Fishing is a secondary profession, which means that you can learn it in addition to your main profession. All you need is a fishing rod and some time.
There are several ways to make money from fishing.
At a low level, you can farm Deviate Fish
Deviate Fish: Deviate Fish can be fished from the waters of the Barrens and Wailing Caverns, and can also be caught from pools in the Barrens. These fish can be cooked into tasty Deviate Delight, a fun food that turns the consumer into either a ninja or a pirate for a short time. Both the fish and the food sell for a very good price.
At level 30, you can farm wreckage in Stranglethorn Vale on the coast.
Not only can you farm wreckage. There are also Firefin Snapper schools and Oily Blackmouth. All sell for a very good price or can be used for alchemy.
Oily Blackmouth: You can fish for Oily Blackmouth in all of Azeroth's coastal waters, but they are most commonly found in the waters of the Eastern Kingdoms. These fish are used by alchemists to make Water Breathing Elixir and other useful potions, so they can be quite valuable.
Firefin Snappers can be fished from Firefin Snapper Schools, which can be found in the coastal waters of the Eastern Kingdoms, particularly in areas such as the coast of Stranglethorn Vale and the southern coast of Stranglethorn Cape. Firefin Snappers are used by cooks to make a variety of dishes, including Firefin Snapper Cakes and Hot Smoked Bass, which are popular with players.
From level 40, you can farm wreckage on the coast of Tanaris.
You can get very good green items to sell or use yourself.
You can also farm Nightfin Snapper in Feralas.
Nightfin Snapper: Nightfin Snapper can be found in the freshwater inland areas of Azeroth, such as lakes and rivers. They are most commonly found in the waters of Feralas and Azshara. These fish are used to make Nightfin Soup, a popular food buff for raiders.
At level 50 and above, you can farm Stonescale Eel and Nightfin Snapper off the coast of Azshara.
Stonescale eel: Stonescale eels can be fished in the coastal waters of both the Eastern Kingdoms and Kalimdor, but they are most commonly found in the waters around Azshara. These fish are used to make Stonescale Oil, which is used by both alchemists and engineers and sells for a very high price.
Of course, the prices of different items and fish can change over time, in different phases, and on different servers. Always check what works best for you.