if ur talking about the old rod then go in the pokemon center on route 32(south of violet city) and talk to someone( i forget who.probably a fisherman dude wearing orange)
Chat with our AI personalities
Fishing in Vermillion with a Super Rod.
one of the fisherman on route 32 will give you a fishing rod.
near lavender town's route
You can get a Magikarp by fishing in any body of water with an Old Rod
explore a bridge in kanto