How do you catch Bulbasaur in Pokemon Yellow?

Go to Cerulean City and beat Misty(There shouldn't be a problem since your Pikachu should know ThunderShock, and ThunderBolt and/or Thunder if you've been training a lot). Then go to the house next door to the Pokemon Center. Go inside, and talk to the girl in the back. She'll ask you to take care of the Bulbasaur beside her. Accept it and she'll give you the Bulbasaur.