Things You'll Need
Instructions
Approach the shore of a body of water until you are at its edge.
Activate a water-type Pokémon's "Surf" ability to ride it into the water.
Move randomly around the body of water to encourage encounters with Pokémon. Although there is an appreciable variety of water Pokémon that you may run into, you can retreat from them until locating a Magikarp, easily one of the most common water Pokémon to run into in a body of water.
Throw a Pokéball at Magikarp to capture him. He will likely not put up much of a struggle in the fight, since his only ability is the inconsequential "Splash" ability.
Select your "Rare Candy" from your inventory and apply it to Magikarp on the list of Pokémon currently with the game's protagonist. Do this until he reaches Level 20, at which point he will evolve into a Gyarados.