Instructions
Purchase a few Poke Balls for 200 Pokedollars each at the PokeMart in Oldale Town.
Head west from Oldale Town onto Route 102. Walk around any patch of tall grass along the route until you encounter a wild Ralts. At the beginning of the game, when you and your lead Pokemon do not have a strong connection yet, Ralts has only a 4% chance of appearing in the wild. However, later on in the game, as you develop a stronger connection with your lead Pokemon, your chance of encountering Ralts will increase.
Engage Ralts in a Pokemon battle. Weaken Ralts until its hit points are in the critical red zone, and then throw a Poke Ball. If it breaks free from the Poke Ball, continue throwing Poke Balls until you capture it.