Fishing is not yet available as a playable activity in the documented version. There is no confirmed process for equipping a rod, casting bait and catching fish.
Resources in the current version
To gather materials, check each item's source in the catalog. Quests, creature loot and merchant offers are existing options. Interacting with a river or lake scene does not guarantee resources.
Where to continue
Use the materials catalog to choose the resource you need and the trade page to understand purchases and exchanges with NPCs. Equipment crafting already has its own guide.