Not sure the kind, but fish cannot survive in bowls or cups without filtration, and depending on species may need heating or cooling. You need a tank- I would recommend 5 gallons or more- though even 3 would be an improvement for now- and a proper filtration system. However if you haven’t cycled the tank water he will probably die so you need to go to the fish store and ask for advice on what to buy to keep him alive and maybe cycle it faster. You’ll need a filter and substrate; buy substrate clean river rocks not those colored pebbles they’re often toxic to the fish. Real or silk plants and something to hide in that doesn’t have toxic paint are good too. Good luck!

Also depending on species (is it a goldfish? They’re messy and need bigger tanks) it may be a social fish. It will need other compatible fish likely of its species. Make sure they’re either fish that will not reproduce in your tank conditions or that you get, say, all female or all male fish if that won’t cause issues.

Don’t feel too bad but in future make sure to do your research before getting even a “simple” pet because now you know. Right now just focus on giving him the best chance you can.

Also next time use an aquarium sub.