First get the old rod. ( because you only need magicarp) empty your party so you only have one pokemon. use the old rod to catch 5 magacarp. Fly to Vermilion and head to the right on rute 11. When you get to the water, head down and there will be a house. go inside with a magacarp as you lead pokemon in your party. Show the fisherman and he will give you a net ball. you can do this repeatedly but they have to get progressivly bigger.