To get Bulbasaur, you need the following requirements-
1. You must beat Misty of Cerulean Gym (Lv18 Staryu,Lv20 Starmie)
2. Talk to your Pikachu. If it jumps twice (which shows its good affection towards you), talk to the girl beside Bulbasaur.
And....Bulbasaur's all yours!