Instructions
Walk around areas where wild Pokémon roam, until you are attacked by an untrained creature. These areas include tall grass, caves, water and certain buildings. Tall grass is the most common Pokémon habitat and can be found outside most towns and along most routes within the game.
Weaken the attacking Pokémon by using your own Pokémon's attacks against it. Do not lower the attacking creature's HP to 0, or else it will faint and become impossible to catch. Use moves that cause "Sleep" or "Paralyze" effects, if your current Pokémon is able to do so. Stop attacking when the Pokémon's HP bar is lowered enough that it turns red. If the Pokémon does faint, continue to search for other wild Pokémon.
Select "Items" from the battle menu. Choose a Pokéball to throw at the Pokémon. If the Pokémon is sufficiently weakened, it will remain in the ball and join your Pokémon collection. If you do not have any Pokéballs, return to the nearest town and purchase some from the Item Store.
Visit any Pokémon Center in any of the towns within the game. Access the PC that is against the wall, and select "Bill's PC" to view your stored Pokémon. Deposit any unwanted Pokémon in your currently active party. Withdraw any Pokémon you wish to add to your party.