If you see a fish, you can catch it. No fish to be seen, can't catch it. many games spawn fish where you start fishing. Valheim does not: it spawns fish and you need to see/find them.

you fish with your fishing rod and the correct bait.
its a mess, but basically:
- buy or find a rod (buy from vendor)
- buy bait from vendor.
- eat stamina food and fish in meadows with vendor bait for a while to level your skill.
- catch something that makes black forest bait. repeat for each biome.